openjdk/test/hotspot/jtreg/compiler/lib/template_framework/Template.java

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package compiler.lib.template_framework;

import java.util.function.BiFunction;
import java.util.function.Function;
import java.util.function.Supplier;

import java.util.List;

import compiler.lib.compile_framework.CompileFramework;
import compiler.lib.ir_framework.TestFramework;

/**
 * The Template Framework allows the generation of code with Templates. The goal is that these Templates are
 * easy to write, and allow regression tests to cover a larger scope, and to make template based fuzzing easy
 * to extend.
 *
 * <p>
 * <strong>Motivation:</strong> We want to make it easy to generate variants of tests. Often, we would like to
 * have a set of tests, corresponding to a set of types, a set of operators, a set of constants, etc. Writing all
 * the tests by hand is cumbersome or even impossible. When generating such tests with scripts, it would be
 * preferable if the code generation happens automatically, and the generator script was checked into the code
 * base. Code generation can go beyond simple regression tests, and one might want to generate random code from
 * a list of possible templates, to fuzz individual Java features and compiler optimizations.
 *
 * <p>
 * The Template Framework provides a facility to generate code with Templates. A Template is essentially a list
 * of tokens that are concatenated (i.e. rendered) to a {@link String}. The Templates can have "holes", which are
 * filled (replaced) by different values at each Template instantiation. For example, these "holes" can
 * be filled with different types, operators or constants. Templates can also be nested, allowing a modular
 * use of Templates.
 *
 * <p>
 * Once we rendered the source code to a {@link String}, we can compile it with the {@link CompileFramework}.
 *
 * <p>
 * <strong>Example:</strong>
 * The following snippets are from the example test {@code TestAdvanced.java}.
 * First, we define a template that generates a {@code @Test} method for a given type, operator and
 * constant generator. We define two constants {@code con1} and {@code con2}, and then use a multiline
 * string with hashtags {@code #} (i.e. "holes")  that are then replaced by the template arguments and the
 * {@link #let} definitions.
 *
 * <p>
 * {@snippet lang=java :
 * var testTemplate = Template.make("typeName", "operator", "generator", (String typeName, String operator, MyGenerator generator) -> body(
 *     let("con1", generator.next()),
 *     let("con2", generator.next()),
 *     """
 *     // #typeName #operator #con1 #con2
 *     public static #typeName $GOLD = $test();
 *
 *     @Test
 *     public static #typeName $test() {
 *         return (#typeName)(#con1 #operator #con2);
 *     }
 *
 *     @Check(test = "$test")
 *     public static void $check(#typeName result) {
 *         Verify.checkEQ(result, $GOLD);
 *     }
 *     """
 * ));
 * }
 *
 * <p>
 * To get an executable test, we define a {@link Template} that produces a class body with a main method. The Template
 * takes a list of types, and calls the {@code testTemplate} defined above for each type and operator. We use
 * the {@link TestFramework} to call our {@code @Test} methods.
 *
 * <p>
 * {@snippet lang=java :
 * var classTemplate = Template.make("types", (List<Type> types) -> body(
 *     let("classpath", comp.getEscapedClassPathOfCompiledClasses()),
 *     """
 *     package p.xyz;
 *
 *     import compiler.lib.ir_framework.*;
 *     import compiler.lib.verify.*;
 *
 *     public class InnerTest {
 *         public static void main() {
 *             // Set the classpath, so that the TestFramework test VM knows where
 *             // the CompileFramework put the class files of the compiled source code.
 *             TestFramework framework = new TestFramework(InnerTest.class);
 *             framework.addFlags("-classpath", "#classpath");
 *             framework.start();
 *         }
 *
 *     """,
 *     // Call the testTemplate for each type and operator, generating a
 *     // list of lists of TemplateToken:
 *     types.stream().map((Type type) ->
 *         type.operators().stream().map((String operator) ->
 *             testTemplate.asToken(type.name(), operator, type.generator())).toList()
 *     ).toList(),
 *     """
 *     }
 *     """
 * ));
 * }
 *
 * <p>
 * Finally, we generate the list of types, and pass it to the class template:
 *
 * <p>
 * {@snippet lang=java :
 * List<Type> types = List.of(
 *     new Type("byte",   GEN_BYTE::next,   List.of("+", "-", "*", "&", "|", "^")),
 *     new Type("char",   GEN_CHAR::next,   List.of("+", "-", "*", "&", "|", "^")),
 *     new Type("short",  GEN_SHORT::next,  List.of("+", "-", "*", "&", "|", "^")),
 *     new Type("int",    GEN_INT::next,    List.of("+", "-", "*", "&", "|", "^")),
 *     new Type("long",   GEN_LONG::next,   List.of("+", "-", "*", "&", "|", "^")),
 *     new Type("float",  GEN_FLOAT::next,  List.of("+", "-", "*", "/")),
 *     new Type("double", GEN_DOUBLE::next, List.of("+", "-", "*", "/"))
 * );
 *
 * // Use the template with one argument, and render it to a String.
 * return classTemplate.render(types);
 * }
 *
 * <p>
 * <strong>Details:</strong>
 * <p>
 * A {@link Template} can have zero or more arguments. A template can be created with {@code make} methods like
 * {@link Template#make(String, Function)}. For each number of arguments there is an implementation
 * (e.g. {@link Template.TwoArgs} for two arguments). This allows the use of generics for the
 * {@link Template} argument types which enables type checking of the {@link Template} arguments.
 *  It is currently only allowed to use up to three arguments.
 *
 * <p>
 * A {@link Template} can be rendered to a {@link String} (e.g. {@link Template.ZeroArgs#render()}).
 * Alternatively, we can generate a {@link Token} (more specifically, a {@link TemplateToken}) with {@code asToken()}
 * (e.g. {@link Template.ZeroArgs#asToken()}), and use the {@link Token} inside another {@link Template#body}.
 *
 * <p>
 * Ideally, we would have used <a href="https://openjdk.org/jeps/430">string templates</a> to inject these Template
 * arguments into the strings. But since string templates are not (yet) available, the Templates provide
 * <strong>hashtag replacements</strong> in the {@link String}s: the Template argument names are captured, and
 * the argument values automatically replace any {@code "#name"} in the {@link String}s. See the different overloads
 * of {@link #make} for examples. Additional hashtag replacements can be defined with {@link #let}.
 *
 * <p>
 * When using nested Templates, there can be collisions with identifiers (e.g. variable names and method names).
 * For this, Templates provide <strong>dollar replacements</strong>, which automatically rename any
 * {@code "$name"} in the {@link String} with a {@code "name_ID"}, where the {@code "ID"} is unique for every use of
 * a Template. The dollar replacement can also be captured with {@link #$}, and passed to nested
 * Templates, which allows sharing of these identifier names between Templates.
 *
 * <p>
 * The dollar and hashtag names must have at least one character. The first character must be a letter
 * or underscore (i.e. {@code a-zA-Z_}), the other characters can also be digits (i.e. {@code a-zA-Z0-9_}).
 * One can use them with or without curly braces, e.g. {@code #name}, {@code #{name}}, {@code $name}, or
 * {@code #{name}}.
 *
 * <p>
 * A {@link TemplateToken} cannot just be used in {@link Template#body}, but it can also be
 * {@link Hook#insert}ed to where a {@link Hook} was {@link Hook#anchor}ed earlier (in some outer scope of the code).
 * For example, while generating code in a method, one can reach out to the scope of the class, and insert a
 * new field, or define a utility method.
 *
 * <p>
 * A {@link TemplateBinding} allows the recursive use of Templates. With the indirection of such a binding,
 * a Template can reference itself.
 *
 * <p>
 * The writer of recursive {@link Template}s must ensure that this recursion terminates. To unify the
 * approach across {@link Template}s, we introduce the concept of {@link #fuel}. Templates are rendered starting
 * with a limited amount of {@link #fuel} (default: 100, see {@link #DEFAULT_FUEL}), which is decreased at each
 * Template nesting by a certain amount (default: 10, see {@link #DEFAULT_FUEL_COST}). The default fuel for a
 * template can be changed when we {@code render()} it (e.g. {@link ZeroArgs#render(float)}) and the default
 * fuel cost with {@link #setFuelCost}) when defining the {@link #body(Object...)}. Recursive templates are
 * supposed to terminate once the {@link #fuel} is depleted (i.e. reaches zero).
 *
 * <p>
 * Code generation can involve keeping track of fields and variables, as well as the scopes in which they
 * are available, and if they are mutable or immutable. We model fields and variables with {@link DataName}s,
 * which we can add to the current scope with {@link #addDataName}. We can access the {@link DataName}s with
 * {@link #dataNames}. We can filter for {@link DataName}s of specific {@link DataName.Type}s, and then
 * we can call {@link DataName.FilteredSet#count}, {@link DataName.FilteredSet#sample},
 * {@link DataName.FilteredSet#toList}, etc. There are many use-cases for this mechanism, especially
 * facilitating communication between the code of outer and inner {@link Template}s. Especially for fuzzing,
 * it may be useful to be able to add fields and variables, and sample them randomly, to create a random data
 * flow graph.
 *
 * <p>
 * Similarly, we may want to model method and class names, and possibly other structural names. We model
 * these names with {@link StructuralName}, which works analogously to {@link DataName}, except that they
 * are not concerned about mutability.
 *
 * <p>
 * When working with {@link DataName}s and {@link StructuralName}s, it is important to be aware of the
 * relevant scopes, as well as the execution order of the {@link Template} lambdas and the evaluation
 * of the {@link Template#body} tokens. When a {@link Template} is rendered, its lambda is invoked. In the
 * lambda, we generate the tokens, and create the {@link Template#body}. Once the lambda returns, the
 * tokens are evaluated one by one. While evaluating the tokens, the {@link Renderer} might encounter a nested
 * {@link TemplateToken}, which in turn triggers the evaluation of that nested {@link Template}, i.e.
 * the evaluation of its lambda and later the evaluation of its tokens. It is important to keep in mind
 * that the lambda is always executed first, and the tokens are evaluated afterwards. A method like
 * {@code dataNames(MUTABLE).exactOf(type).count()} is a method that is executed during the evaluation
 * of the lambda. But a method like {@link #addDataName} returns a token, and does not immediately add
 * the {@link DataName}. This ensures that the {@link DataName} is only inserted when the tokens are
 * evaluated, so that it is inserted at the exact scope where we would expect it.
 *
 * <p>
 * Let us look at the following example to better understand the execution order.
 *
 * <p>
 * {@snippet lang=java :
 * var testTemplate = Template.make(() -> body(
 *     // The lambda has just been invoked.
 *     // We count the DataNames and assign the count to the hashtag replacement "c1".
 *     let("c1", dataNames(MUTABLE).exactOf(someType).count()),
 *     // We want to define a DataName "v1", and create a token for it.
 *     addDataName($("v1"), someType, MUTABLE),
 *     // We count the DataNames again, but the count does NOT change compared to "c1".
 *     // This is because the token for "v1" is only evaluated later.
 *     let("c2", dataNames(MUTABLE).exactOf(someType).count()),
 *     // Create a nested scope.
 *     METHOD_HOOK.anchor(
 *         // We want to define a DataName "v2", which is only valid inside this
 *         // nested scope.
 *         addDataName($("v2"), someType, MUTABLE),
 *         // The count is still not different to "c1".
 *         let("c3", dataNames(MUTABLE).exactOf(someType).count()),
 *         // We nest a Template. This creates a TemplateToken, which is later evaluated.
 *         // By the time the TemplateToken is evaluated, the tokens from above will
 *         // be already evaluated. Hence, "v1" and "v2" are added by then, and if the
 *         // "otherTemplate" were to count the DataNames, the count would be increased
 *         // by 2 compared to "c1".
 *         otherTemplate.asToken()
 *     ),
 *     // After closing the scope, "v2" is no longer available.
 *     // The count is still the same as "c1", since "v1" is still only a token.
 *     let("c4", dataNames(MUTABLE).exactOf(someType).count()),
 *     // We nest another Template. Again, this creates a TemplateToken, which is only
 *     // evaluated later. By that time, the token for "v1" is evaluated, and so the
 *     // nested Template would observe an increment in the count.
 *     anotherTemplate.asToken()
 *     // By this point, all methods are called, and the tokens generated.
 *     // The lambda returns the "body", which is all of the tokens that we just
 *     // generated. After returning from the lambda, the tokens will be evaluated
 *     // one by one.
 * ));
 * }

 * <p>
 * More examples for these functionalities can be found in {@code TestTutorial.java}, {@code TestSimple.java},
 * and {@code TestAdvanced.java}, which all produce compilable Java code. Additional examples can be found in
 * the tests, such as {@code TestTemplate.java} and {@code TestFormat.java}, which do not necessarily generate
 * valid Java code, but generate deterministic Strings which are easier to verify, and may also serve as a
 * reference when learning about these functionalities.
 */
public sealed interface Template permits Template.ZeroArgs,
                                         Template.OneArg,
                                         Template.TwoArgs,
                                         Template.ThreeArgs {

    /**
     * A {@link Template} with no arguments.
     *
     * @param function The {@link Supplier} that creates the {@link TemplateBody}.
     */
    record ZeroArgs(Supplier<TemplateBody> function) implements Template {
        TemplateBody instantiate() {
            return function.get();
        }

        /**
         * Creates a {@link TemplateToken} which can be used as a {@link Token} inside
         * a {@link Template} for nested code generation.
         *
         * @return The {@link TemplateToken} to use the {@link Template} inside another
         *         {@link Template}.
         */
        public TemplateToken asToken() {
            return new TemplateToken.ZeroArgs(this);
        }

        /**
         * Renders the {@link Template} to a {@link String}.
         *
         * @return The {@link String}, resulting from rendering the {@link Template}.
         */
        public String render() {
            return new TemplateToken.ZeroArgs(this).render();
        }

        /**
         * Renders the {@link Template} to a {@link String}.
         *
         * @param fuel The amount of fuel provided for recursive Template instantiations.
         * @return The {@link String}, resulting from rendering the {@link Template}.
         */
        public String render(float fuel) {
            return new TemplateToken.ZeroArgs(this).render(fuel);
        }
    }

    /**
     * A {@link Template} with one argument.
     *
     * @param arg1Name The name of the (first) argument, used for hashtag replacements in the {@link Template}.
     * @param <T1> The type of the (first) argument.
     * @param function The {@link Function} that creates the {@link TemplateBody} given the template argument.
     */
    record OneArg<T1>(String arg1Name, Function<T1, TemplateBody> function) implements Template {
        TemplateBody instantiate(T1 arg1) {
            return function.apply(arg1);
        }

        /**
         * Creates a {@link TemplateToken} which can be used as a {@link Token} inside
         * a {@link Template} for nested code generation.
         *
         * @param arg1 The value for the (first) argument.
         * @return The {@link TemplateToken} to use the {@link Template} inside another
         *         {@link Template}.
         */
        public TemplateToken asToken(T1 arg1) {
            return new TemplateToken.OneArg<>(this, arg1);
        }

        /**
         * Renders the {@link Template} to a {@link String}.
         *
         * @param arg1 The value for the first argument.
         * @return The {@link String}, resulting from rendering the {@link Template}.
         */
        public String render(T1 arg1) {
            return new TemplateToken.OneArg<>(this, arg1).render();
        }

        /**
         * Renders the {@link Template} to a {@link String}.
         *
         * @param arg1 The value for the first argument.
         * @param fuel The amount of fuel provided for recursive Template instantiations.
         * @return The {@link String}, resulting from rendering the {@link Template}.
         */
        public String render(float fuel, T1 arg1) {
            return new TemplateToken.OneArg<>(this, arg1).render(fuel);
        }
    }

    /**
     * A {@link Template} with two arguments.
     *
     * @param arg1Name The name of the first argument, used for hashtag replacements in the {@link Template}.
     * @param arg2Name The name of the second argument, used for hashtag replacements in the {@link Template}.
     * @param <T1> The type of the first argument.
     * @param <T2> The type of the second argument.
     * @param function The {@link BiFunction} that creates the {@link TemplateBody} given the template arguments.
     */
    record TwoArgs<T1, T2>(String arg1Name, String arg2Name, BiFunction<T1, T2, TemplateBody> function) implements Template {
        TemplateBody instantiate(T1 arg1, T2 arg2) {
            return function.apply(arg1, arg2);
        }

        /**
         * Creates a {@link TemplateToken} which can be used as a {@link Token} inside
         * a {@link Template} for nested code generation.
         *
         * @param arg1 The value for the first argument.
         * @param arg2 The value for the second argument.
         * @return The {@link TemplateToken} to use the {@link Template} inside another
         *         {@link Template}.
         */
        public TemplateToken asToken(T1 arg1, T2 arg2) {
            return new TemplateToken.TwoArgs<>(this, arg1, arg2);
        }

        /**
         * Renders the {@link Template} to a {@link String}.
         *
         * @param arg1 The value for the first argument.
         * @param arg2 The value for the second argument.
         * @return The {@link String}, resulting from rendering the {@link Template}.
         */
        public String render(T1 arg1, T2 arg2) {
            return new TemplateToken.TwoArgs<>(this, arg1, arg2).render();
        }

        /**
         * Renders the {@link Template} to a {@link String}.
         *
         * @param arg1 The value for the first argument.
         * @param arg2 The value for the second argument.
         * @param fuel The amount of fuel provided for recursive Template instantiations.
         * @return The {@link String}, resulting from rendering the {@link Template}.
         */
        public String render(float fuel, T1 arg1, T2 arg2) {
            return new TemplateToken.TwoArgs<>(this, arg1, arg2).render(fuel);
        }
    }

    /**
     * Interface for function with three arguments.
     *
     * @param <T> Type of the first argument.
     * @param <U> Type of the second argument.
     * @param <V> Type of the third argument.
     * @param <R> Type of the return value.
     */
    @FunctionalInterface
    interface TriFunction<T, U, V, R> {

        /**
         * Function definition for the three argument functions.
         *
         * @param t The first argument.
         * @param u The second argument.
         * @param v The third argument.
         * @return Return value of the three argument function.
         */
        R apply(T t, U u, V v);
    }

    /**
     * A {@link Template} with three arguments.
     *
     * @param arg1Name The name of the first argument, used for hashtag replacements in the {@link Template}.
     * @param arg2Name The name of the second argument, used for hashtag replacements in the {@link Template}.
     * @param arg3Name The name of the third argument, used for hashtag replacements in the {@link Template}.
     * @param <T1> The type of the first argument.
     * @param <T2> The type of the second argument.
     * @param <T3> The type of the third argument.
     * @param function The function with three arguments that creates the {@link TemplateBody} given the template arguments.
     */
    record ThreeArgs<T1, T2, T3>(String arg1Name, String arg2Name, String arg3Name, TriFunction<T1, T2, T3, TemplateBody> function) implements Template {
        TemplateBody instantiate(T1 arg1, T2 arg2, T3 arg3) {
            return function.apply(arg1, arg2, arg3);
        }

        /**
         * Creates a {@link TemplateToken} which can be used as a {@link Token} inside
         * a {@link Template} for nested code generation.
         *
         * @param arg1 The value for the first argument.
         * @param arg2 The value for the second argument.
         * @param arg3 The value for the third argument.
         * @return The {@link TemplateToken} to use the {@link Template} inside another
         *         {@link Template}.
         */
        public TemplateToken asToken(T1 arg1, T2 arg2, T3 arg3) {
            return new TemplateToken.ThreeArgs<>(this, arg1, arg2, arg3);
        }

        /**
         * Renders the {@link Template} to a {@link String}.
         *
         * @param arg1 The value for the first argument.
         * @param arg2 The value for the second argument.
         * @param arg3 The value for the third argument.
         * @return The {@link String}, resulting from rendering the {@link Template}.
         */
        public String render(T1 arg1, T2 arg2, T3 arg3) {
            return new TemplateToken.ThreeArgs<>(this, arg1, arg2, arg3).render();
        }

        /**
         * Renders the {@link Template} to a {@link String}.
         *
         * @param arg1 The value for the first argument.
         * @param arg2 The value for the second argument.
         * @param arg3 The value for the third argument.
         * @param fuel The amount of fuel provided for recursive Template instantiations.
         * @return The {@link String}, resulting from rendering the {@link Template}.
         */
        public String render(float fuel, T1 arg1, T2 arg2, T3 arg3) {
            return new TemplateToken.ThreeArgs<>(this, arg1, arg2, arg3).render(fuel);
        }
    }

    /**
     * Creates a {@link Template} with no arguments.
     * See {@link #body} for more details about how to construct a Template with {@link Token}s.
     *
     * <p>
     * Example:
     * {@snippet lang=java :
     * var template = Template.make(() -> body(
     *     """
     *     Multi-line string or other tokens.
     *     """
     * ));
     * }
     *
     * @param body The {@link TemplateBody} created by {@link Template#body}.
     * @return A {@link Template} with zero arguments.
     */
    static Template.ZeroArgs make(Supplier<TemplateBody> body) {
        return new Template.ZeroArgs(body);
    }

    /**
     * Creates a {@link Template} with one argument.
     * See {@link #body} for more details about how to construct a Template with {@link Token}s.
     * Good practice but not enforced but not enforced: {@code arg1Name} should match the lambda argument name.
     *
     * <p>
     * Here is an example with template argument {@code 'a'}, captured once as string name
     * for use in hashtag replacements, and captured once as lambda argument with the corresponding type
     * of the generic argument.
     * {@snippet lang=java :
     * var template = Template.make("a", (Integer a) -> body(
     *     """
     *     Multi-line string or other tokens.
     *     We can use the hashtag replacement #a to directly insert the String value of a.
     *     """,
     *     "We can also use the captured parameter of a: " + a
     * ));
     * }
     *
     * @param body The {@link TemplateBody} created by {@link Template#body}.
     * @param <T1> Type of the (first) argument.
     * @param arg1Name The name of the (first) argument for hashtag replacement.
     * @return A {@link Template} with one argument.
     */
    static <T1> Template.OneArg<T1> make(String arg1Name, Function<T1, TemplateBody> body) {
        return new Template.OneArg<>(arg1Name, body);
    }

    /**
     * Creates a {@link Template} with two arguments.
     * See {@link #body} for more details about how to construct a Template with {@link Token}s.
     * Good practice but not enforced: {@code arg1Name} and {@code arg2Name} should match the lambda argument names.
     *
     * <p>
     * Here is an example with template arguments {@code 'a'} and {@code 'b'}, captured once as string names
     * for use in hashtag replacements, and captured once as lambda arguments with the corresponding types
     * of the generic arguments.
     * {@snippet lang=java :
     * var template = Template.make("a", "b", (Integer a, String b) -> body(
     *     """
     *     Multi-line string or other tokens.
     *     We can use the hashtag replacement #a and #b to directly insert the String value of a and b.
     *     """,
     *     "We can also use the captured parameter of a and b: " + a + " and " + b
     * ));
     * }
     *
     * @param body The {@link TemplateBody} created by {@link Template#body}.
     * @param <T1> Type of the first argument.
     * @param arg1Name The name of the first argument for hashtag replacement.
     * @param <T2> Type of the second argument.
     * @param arg2Name The name of the second argument for hashtag replacement.
     * @return A {@link Template} with two arguments.
     */
    static <T1, T2> Template.TwoArgs<T1, T2> make(String arg1Name, String arg2Name, BiFunction<T1, T2, TemplateBody> body) {
        return new Template.TwoArgs<>(arg1Name, arg2Name, body);
    }

    /**
     * Creates a {@link Template} with three arguments.
     * See {@link #body} for more details about how to construct a Template with {@link Token}s.
     * Good practice but not enforced: {@code arg1Name}, {@code arg2Name}, and {@code arg3Name} should match the lambda argument names.
     *
     * @param body The {@link TemplateBody} created by {@link Template#body}.
     * @param <T1> Type of the first argument.
     * @param arg1Name The name of the first argument for hashtag replacement.
     * @param <T2> Type of the second argument.
     * @param arg2Name The name of the second argument for hashtag replacement.
     * @param <T3> Type of the third argument.
     * @param arg3Name The name of the third argument for hashtag replacement.
     * @return A {@link Template} with three arguments.
     */
    static <T1, T2, T3> Template.ThreeArgs<T1, T2, T3> make(String arg1Name, String arg2Name, String arg3Name, Template.TriFunction<T1, T2, T3, TemplateBody> body) {
        return new Template.ThreeArgs<>(arg1Name, arg2Name, arg3Name, body);
    }

    /**
     * Creates a {@link TemplateBody} from a list of tokens, which can be {@link String}s,
     * boxed primitive types (for example {@link Integer} or auto-boxed {@code int}), any {@link Token},
     * or {@link List}s of any of these.
     *
     * <p>
     * {@snippet lang=java :
     * var template = Template.make(() -> body(
     *     """
     *     Multi-line string
     *     """,
     *     "normal string ", Integer.valueOf(3), 3, Float.valueOf(1.5f), 1.5f,
     *     List.of("abc", "def"),
     *     nestedTemplate.asToken(42)
     * ));
     * }
     *
     * @param tokens A list of tokens, which can be {@link String}s, boxed primitive types
     *               (for example {@link Integer}), any {@link Token}, or {@link List}s
     *               of any of these.
     * @return The {@link TemplateBody} which captures the list of validated {@link Token}s.
     * @throws IllegalArgumentException if the list of tokens contains an unexpected object.
     */
    static TemplateBody body(Object... tokens) {
        return new TemplateBody(Token.parse(tokens));
    }

    /**
     * Retrieves the dollar replacement of the {@code 'name'} for the
     * current Template that is being instantiated. It returns the same
     * dollar replacement as the string use {@code "$name"}.
     *
     * <p>
     * Here is an example where a Template creates a local variable {@code 'var'},
     * with an implicit dollar replacement, and then captures that dollar replacement
     * using {@link #$} for the use inside a nested template.
     * {@snippet lang=java :
     * var template = Template.make(() -> body(
     *     """
     *     int $var = 42;
     *     """,
     *     otherTemplate.asToken($("var"))
     * ));
     * }
     *
     * @param name The {@link String} name of the name.
     * @return The dollar replacement for the {@code 'name'}.
     */
    static String $(String name) {
        return Renderer.getCurrent().$(name);
    }

    /**
     * Define a hashtag replacement for {@code "#key"}, with a specific value.
     *
     * <p>
     * {@snippet lang=java :
     * var template = Template.make("a", (Integer a) -> body(
     *     let("b", a * 5),
     *     """
     *     System.out.println("Use a and b with hashtag replacement: #a and #b");
     *     """
     * ));
     * }
     *
     * @param key Name for the hashtag replacement.
     * @param value The value that the hashtag is replaced with.
     * @return A token that does nothing, so that the {@link #let} can easily be put in a list of tokens
     *         inside a {@link Template#body}.
     * @throws RendererException if there is a duplicate hashtag {@code key}.
     */
    static Token let(String key, Object value) {
        Renderer.getCurrent().addHashtagReplacement(key, value);
        return new NothingToken();
    }

    /**
     * Define a hashtag replacement for {@code "#key"}, with a specific value, which is also captured
     * by the provided {@code function} with type {@code <T>}.
     *
     * <p>
     * {@snippet lang=java :
     * var template = Template.make("a", (Integer a) -> let("b", a * 2, (Integer b) -> body(
     *     """
     *     System.out.println("Use a and b with hashtag replacement: #a and #b");
     *     """,
     *     "System.out.println(\"Use a and b as capture variables:\"" + a + " and " + b + ");\n"
     * )));
     * }
     *
     * @param key Name for the hashtag replacement.
     * @param value The value that the hashtag is replaced with.
     * @param <T> The type of the value.
     * @param function The function that is applied with the provided {@code value}.
     * @return A {@link TemplateBody}.
     * @throws RendererException if there is a duplicate hashtag {@code key}.
     */
    static <T> TemplateBody let(String key, T value, Function<T, TemplateBody> function) {
        Renderer.getCurrent().addHashtagReplacement(key, value);
        return function.apply(value);
    }

    /**
     * Default amount of fuel for Template rendering. It guides the nesting depth of Templates. Can be changed when
     * rendering a template with {@code render(fuel)} (e.g. {@link ZeroArgs#render(float)}).
     */
    float DEFAULT_FUEL = 100.0f;

    /**
     * The default amount of fuel spent per Template. It is subtracted from the current {@link #fuel} at every
     * nesting level, and once the {@link #fuel} reaches zero, the nesting is supposed to terminate. Can be changed
     * with {@link #setFuelCost(float)} inside {@link #body(Object...)}.
     */
    float DEFAULT_FUEL_COST = 10.0f;

    /**
     * The current remaining fuel for nested Templates. Every level of Template nesting
     * subtracts a certain amount of fuel, and when it reaches zero, Templates are supposed to
     * stop nesting, if possible. This is not a hard rule, but a guide, and a mechanism to ensure
     * termination in recursive Template instantiations.
     *
     * <p>
     * Example of a recursive Template, which checks the remaining {@link #fuel} at every level,
     * and terminates if it reaches zero. It also demonstrates the use of {@link TemplateBinding} for
     * the recursive use of Templates. We {@link Template.OneArg#render} with {@code 30} total fuel,
     * and spend {@code 5} fuel at each recursion level.
     *
     * <p>
     * {@snippet lang=java :
     * var binding = new TemplateBinding<Template.OneArg<Integer>>();
     * var template = Template.make("depth", (Integer depth) -> body(
     *     setFuelCost(5.0f),
     *     let("fuel", fuel()),
     *     """
     *     System.out.println("Currently at depth #depth with fuel #fuel");
     *     """,
     *     (fuel() > 0) ? binding.get().asToken(depth + 1) :
     *                    "// terminate\n"
     * ));
     * binding.bind(template);
     * String code = template.render(30.0f, 0);
     * }
     *
     * @return The amount of fuel left for nested Template use.
     */
    static float fuel() {
        return Renderer.getCurrent().fuel();
    }

    /**
     * Changes the amount of fuel used for the current Template, where the default is
     * {@link Template#DEFAULT_FUEL_COST}.
     *
     * @param fuelCost The amount of fuel used for the current Template.
     * @return A token for convenient use in {@link Template#body}.
     */
    static Token setFuelCost(float fuelCost) {
        Renderer.getCurrent().setFuelCost(fuelCost);
        return new NothingToken();
    }

    /**
     * Add a {@link DataName} in the current scope, that is the innermost of either
     * {@link Template#body} or {@link Hook#anchor}.
     *
     * @param name The name of the {@link DataName}, i.e. the {@link String} used in code.
     * @param type The type of the {@link DataName}.
     * @param mutability Indicates if the {@link DataName} is to be mutable or immutable,
     *                   i.e. if we intend to use the {@link DataName} only for reading
     *                   or if we also allow it to be mutated.
     * @param weight The weight of the {@link DataName}, which correlates to the probability
     *               of this {@link DataName} being chosen when we sample.
     *               Must be a value from 1 to 1000.
     * @return The token that performs the defining action.
     */
    static Token addDataName(String name, DataName.Type type, DataName.Mutability mutability, int weight) {
        if (mutability != DataName.Mutability.MUTABLE &&
            mutability != DataName.Mutability.IMMUTABLE) {
            throw new IllegalArgumentException("Unexpected mutability: " + mutability);
        }
        boolean mutable = mutability == DataName.Mutability.MUTABLE;
        if (weight <= 0 || 1000 < weight) {
            throw new IllegalArgumentException("Unexpected weight: " + weight);
        }
        return new AddNameToken(new DataName(name, type, mutable, weight));
    }

    /**
     * Add a {@link DataName} in the current scope, that is the innermost of either
     * {@link Template#body} or {@link Hook#anchor}, with a {@code weight} of 1.
     *
     * @param name The name of the {@link DataName}, i.e. the {@link String} used in code.
     * @param type The type of the {@link DataName}.
     * @param mutability Indicates if the {@link DataName} is to be mutable or immutable,
     *                   i.e. if we intend to use the {@link DataName} only for reading
     *                   or if we also allow it to be mutated.
     * @return The token that performs the defining action.
     */
    static Token addDataName(String name, DataName.Type type, DataName.Mutability mutability) {
        return addDataName(name, type, mutability, 1);
    }

    /**
     * Access the set of {@link DataName}s, for sampling, counting, etc.
     *
     * @param mutability Indicates if we only sample from mutable, immutable or either {@link DataName}s.
     * @return A view on the {@link DataName}s, on which we can sample, count, etc.
     */
    static DataName.FilteredSet dataNames(DataName.Mutability mutability) {
        return new DataName.FilteredSet(mutability);
    }

    /**
     * Add a {@link StructuralName} in the current scope, that is the innermost of either
     * {@link Template#body} or {@link Hook#anchor}.
     *
     * @param name The name of the {@link StructuralName}, i.e. the {@link String} used in code.
     * @param type The type of the {@link StructuralName}.
     * @param weight The weight of the {@link StructuralName}, which correlates to the probability
     *               of this {@link StructuralName} being chosen when we sample.
     *               Must be a value from 1 to 1000.
     * @return The token that performs the defining action.
     */
    static Token addStructuralName(String name, StructuralName.Type type, int weight) {
        if (weight <= 0 || 1000 < weight) {
            throw new IllegalArgumentException("Unexpected weight: " + weight);
        }
        return new AddNameToken(new StructuralName(name, type, weight));
    }

    /**
     * Add a {@link StructuralName} in the current scope, that is the innermost of either
     * {@link Template#body} or {@link Hook#anchor}, with a {@code weight} of 1.
     *
     * @param name The name of the {@link StructuralName}, i.e. the {@link String} used in code.
     * @param type The type of the {@link StructuralName}.
     * @return The token that performs the defining action.
     */
    static Token addStructuralName(String name, StructuralName.Type type) {
        return addStructuralName(name, type, 1);
    }

    /**
     * Access the set of {@link StructuralName}s, for sampling, counting, etc.
     *
     * @return A view on the {@link StructuralName}s, on which we can sample, count, etc.
     */
    static StructuralName.FilteredSet structuralNames() {
        return new StructuralName.FilteredSet();
    }
}