137 lines
4.0 KiB
Java
137 lines
4.0 KiB
Java
package com.craftinginterpreters.lox;
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class Interpreter implements Expr.Visitor<Object> {
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void interpret(Expr expression) {
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try {
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Object value = evaluate(expression);
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System.out.println(stringify(value));
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} catch (RuntimeError error) {
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Lox.runtimeError(error);
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}
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}
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@Override
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public Object visitLiteralExpr(Expr.Literal expr) {
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return expr.value;
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}
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@Override
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public Object visitUnaryExpr(Expr.Unary expr) {
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Object right = evaluate(expr.right);
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switch (expr.operator.type) {
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case BANG:
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return !isTruthy(right);
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case MINUS:
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checkNumberOperand(expr.operator, right);
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return -(double) right;
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}
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// Unreachable.
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return null;
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}
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@Override
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public Object visitGroupingExpr(Expr.Grouping expr) {
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return evaluate(expr.expression);
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}
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@Override
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public Object visitBinaryExpr(Expr.Binary expr) {
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Object left = evaluate(expr.left);
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Object right = evaluate(expr.right);
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switch (expr.operator.type) {
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case BANG_EQUAL:
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return !isEqual(left, right);
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case EQUAL_EQUAL:
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return isEqual(left, right);
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case GREATER:
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checkNumberOperands(expr.operator, left, right);
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return (double) left > (double) right;
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case GREATER_EQUAL:
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checkNumberOperands(expr.operator, left, right);
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return (double) left >= (double) right;
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case LESS:
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checkNumberOperands(expr.operator, left, right);
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return (double) left < (double) right;
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case LESS_EQUAL:
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checkNumberOperands(expr.operator, left, right);
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return (double) left <= (double) right;
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case MINUS:
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checkNumberOperands(expr.operator, left, right);
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return (double) left - (double) right;
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case PLUS:
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if (left instanceof Double && right instanceof Double) {
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return (double) left + (double) right;
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}
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if (left instanceof String && right instanceof String) {
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return (String) left + (String) right;
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}
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throw new RuntimeError(expr.operator, "Operands must be two numbers or two strings.");
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case SLASH:
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checkNumberOperands(expr.operator, left, right);
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return (double) left / (double) right;
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case STAR:
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checkNumberOperands(expr.operator, left, right);
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return (double) left * (double) right;
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}
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// Unreachable.
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return null;
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}
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private Object evaluate(Expr expr) {
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return expr.accept(this);
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}
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private void checkNumberOperand(Token operator, Object operand) {
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if (operand instanceof Double)
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return;
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throw new RuntimeError(operator, "Operand must be a number.");
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}
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private void checkNumberOperands(Token operator, Object left, Object right) {
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if (left instanceof Double && right instanceof Double)
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return;
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throw new RuntimeError(operator, "Operands must be numbers.");
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}
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private boolean isTruthy(Object object) {
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if (object == null)
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return false;
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if (object instanceof Boolean)
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return (boolean) object;
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return true;
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}
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private boolean isEqual(Object a, Object b) {
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if (a == null && b == null)
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return true;
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if (a == null)
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return false;
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return a.equals(b);
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}
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private String stringify(Object object) {
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if (object == null)
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return "nil";
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if (object instanceof Double) {
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String text = object.toString();
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if (text.endsWith(".0")) {
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text = text.substring(0, text.length() - 2);
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}
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return text;
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}
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return object.toString();
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}
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}
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