jabba/src/main/java/com/reliancy/io/JsonDecoder.java

package com.reliancy.io;

import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;

/**
 * Lightweight JSON decoder that produces common Java object trees.
 */
public final class JsonDecoder {
    private final CharSequence input;
    private int index;

    private JsonDecoder(CharSequence input) {
        this.input = input == null ? "" : input;
    }

    public static Object decode(CharSequence input) {
        JsonDecoder parser = new JsonDecoder(input);
        Object value = parser.readValue();
        parser.skipWhitespace();
        if (parser.hasMore()) {
            throw parser.error("Unexpected trailing content");
        }
        return value;
    }

    public static Map<String, Object> decodeObject(CharSequence input) {
        Object value = decode(input);
        if (value instanceof Map<?, ?> map) {
            @SuppressWarnings("unchecked")
            Map<String, Object> typed = (Map<String, Object>) map;
            return typed;
        }
        throw new IllegalArgumentException("JSON payload is not an object");
    }

    public static List<Object> decodeArray(CharSequence input) {
        Object value = decode(input);
        if (value instanceof List<?> list) {
            @SuppressWarnings("unchecked")
            List<Object> typed = (List<Object>) list;
            return typed;
        }
        throw new IllegalArgumentException("JSON payload is not an array");
    }

    private Object readValue() {
        skipWhitespace();
        if (!hasMore()) {
            throw error("Unexpected end of input");
        }
        char ch = current();
        switch (ch) {
            case '{':
                return readObject();
            case '[':
                return readArray();
            case '"':
                return readString();
            case 't':
                expectLiteral("true");
                return Boolean.TRUE;
            case 'f':
                expectLiteral("false");
                return Boolean.FALSE;
            case 'n':
                expectLiteral("null");
                return null;
            default:
                if (ch == '-' || Character.isDigit(ch)) {
                    return readNumber();
                }
                throw error("Unexpected token");
        }
    }

    private Map<String, Object> readObject() {
        expect('{');
        LinkedHashMap<String, Object> result = new LinkedHashMap<>();
        skipWhitespace();
        if (peek('}')) {
            index++;
            return result;
        }
        while (true) {
            skipWhitespace();
            String key = readString();
            skipWhitespace();
            expect(':');
            Object value = readValue();
            result.put(key, value);
            skipWhitespace();
            if (peek('}')) {
                index++;
                return result;
            }
            expect(',');
        }
    }

    private List<Object> readArray() {
        expect('[');
        ArrayList<Object> result = new ArrayList<>();
        skipWhitespace();
        if (peek(']')) {
            index++;
            return result;
        }
        while (true) {
            result.add(readValue());
            skipWhitespace();
            if (peek(']')) {
                index++;
                return result;
            }
            expect(',');
        }
    }

    private String readString() {
        expect('"');
        StringBuilder buf = new StringBuilder();
        while (hasMore()) {
            char ch = input.charAt(index++);
            if (ch == '"') {
                return buf.toString();
            }
            if (ch != '\\') {
                buf.append(ch);
                continue;
            }
            if (!hasMore()) {
                throw error("Unterminated escape sequence");
            }
            char esc = input.charAt(index++);
            switch (esc) {
                case '"':
                case '\\':
                case '/':
                    buf.append(esc);
                    break;
                case 'b':
                    buf.append('\b');
                    break;
                case 'f':
                    buf.append('\f');
                    break;
                case 'n':
                    buf.append('\n');
                    break;
                case 'r':
                    buf.append('\r');
                    break;
                case 't':
                    buf.append('\t');
                    break;
                case 'u':
                    buf.append(readUnicodeEscape());
                    break;
                default:
                    throw error("Unsupported escape sequence");
            }
        }
        throw error("Unterminated string");
    }

    private Number readNumber() {
        int start = index;
        if (peek('-')) {
            index++;
        }
        readDigits();
        boolean fractional = false;
        if (peek('.')) {
            fractional = true;
            index++;
            readDigits();
        }
        if (peek('e') || peek('E')) {
            fractional = true;
            index++;
            if (peek('+') || peek('-')) {
                index++;
            }
            readDigits();
        }
        String token = input.subSequence(start, index).toString();
        if (!Handy.isNumeric(token)) {
            throw error("Invalid number");
        }
        if (fractional) {
            return Double.valueOf(token);
        }
        try {
            return Integer.valueOf(token);
        } catch (NumberFormatException ignore) {
            return Long.valueOf(token);
        }
    }

    private char readUnicodeEscape() {
        if (index + 4 > input.length()) {
            throw error("Invalid unicode escape");
        }
        int value = 0;
        for (int i = 0; i < 4; i++) {
            char ch = input.charAt(index++);
            int digit = Character.digit(ch, 16);
            if (digit < 0) {
                throw error("Invalid unicode escape");
            }
            value = (value << 4) + digit;
        }
        return (char) value;
    }

    private void readDigits() {
        int start = index;
        while (hasMore() && Character.isDigit(current())) {
            index++;
        }
        if (start == index) {
            throw error("Expected digit");
        }
    }

    private void expectLiteral(String literal) {
        for (int i = 0; i < literal.length(); i++) {
            if (!hasMore() || input.charAt(index++) != literal.charAt(i)) {
                throw error("Expected literal " + literal);
            }
        }
    }

    private void expect(char expected) {
        skipWhitespace();
        if (!peek(expected)) {
            throw error("Expected '" + expected + "'");
        }
        index++;
    }

    private void skipWhitespace() {
        while (hasMore() && Character.isWhitespace(current())) {
            index++;
        }
    }

    private boolean hasMore() {
        return index < input.length();
    }

    private boolean peek(char ch) {
        return hasMore() && current() == ch;
    }

    private char current() {
        return input.charAt(index);
    }

    private IllegalArgumentException error(String message) {
        return new IllegalArgumentException(message + " at offset " + index);
    }
}