package org.json;

import static java.lang.String.format;
import static java.util.Collections.emptyList;

import java.io.UnsupportedEncodingException;
import java.net.URLDecoder;
import java.util.ArrayList;
import java.util.List;

/**
 * A JSON Pointer is a simple query language defined for JSON documents by
 * <a href="https://tools.ietf.org/html/rfc6901">RFC 6901</a>. 
 */
public class JSONPointer {

    private List<String> refTokens;

    /**
     * Pre-parses and initializes a new {@code JSONPointer} instance. If you want to
     * evaluate the same JSON Pointer on different JSON documents then it is recommended
     * to keep the {@code JSONPointer} instances due to performance considerations.
     * 
     * @param pointer the JSON String or URI Fragment representation of the JSON pointer.
     * @throws IllegalArgumentException if {@code pointer} is not a valid JSON pointer
     */
    public JSONPointer(String pointer) {
        if (pointer == null) {
            throw new NullPointerException("pointer cannot be null");
        }
        if (pointer.isEmpty()) {
            refTokens = emptyList();
            return;
        }
        if (pointer.startsWith("#/")) {
            pointer = pointer.substring(2);
            try {
                pointer = URLDecoder.decode(pointer, "utf-8");
            } catch (UnsupportedEncodingException e) {
                throw new RuntimeException(e);
            }
        } else if (pointer.startsWith("/")) {
            pointer = pointer.substring(1);
        } else {
            throw new IllegalArgumentException("a JSON pointer should start with '/' or '#/'");
        }
        refTokens = new ArrayList<String>();
        for (String token : pointer.split("/")) {
            refTokens.add(unescape(token));
        }
    }

    private String unescape(String token) {
        return token.replace("~1", "/").replace("~0", "~")
                .replace("\\\"", "\"")
                .replace("\\\\", "\\");
    }

    /**
     * Evaluates this JSON Pointer on the given {@code document}. The {@code document}
     * is usually a {@link JSONObject} or a {@link JSONArray} instance, but the empty
     * JSON Pointer ({@code ""}) can be evaluated on any JSON values and in such case the
     * returned value will be {@code document} itself. 
     * 
     * @param document the JSON document which should be the subject of querying.
     * @return the result of the evaluation
     * @throws JSONPointerException if an error occurs during evaluation
     */
    public Object queryFrom(Object document) {
        if (refTokens.isEmpty()) {
            return document;
        }
        Object current = document;
        for (String token : refTokens) {
            if (current instanceof JSONObject) {
                current = ((JSONObject) current).opt(unescape(token));
            } else if (current instanceof JSONArray) {
                current = readByIndexToken(current, token);
            } else {
                throw new JSONPointerException(format(
                        "value [%s] is not an array or object therefore its key %s cannot be resolved", current,
                        token));
            }
        }
        return current;
    }

    private Object readByIndexToken(Object current, String indexToken) {
        try {
            int index = Integer.parseInt(indexToken);
            JSONArray currentArr = (JSONArray) current;
            if (index >= currentArr.length()) {
                throw new JSONPointerException(format("index %d is out of bounds - the array has %d elements", index,
                        currentArr.length()));
            }
            return currentArr.get(index);
        } catch (NumberFormatException e) {
            throw new JSONPointerException(format("%s is not an array index", indexToken), e);
        }
    }
}