/**
 * An implementation of a multi-valued mapping, where one key may map to one
 * or more values.
 */
module http_primitives.multivalue_map;

import http_primitives.optional;

/**
 * A multi-valued mapping, where a key is mapped to one or more values. The map
 * can optionally be sorted by keys for O(log(n)) lookup and retrieval, and
 * O(n*log(n)) insertion, instead of the default linear search.
 */
struct MultiValueMap(KeyType, ValueType, bool Sorted = true, alias KeySort = (a, b) => a < b) {
    /// The internal structure used to store each key and set of values.
    static struct Entry {
        /// The key for this entry.
        KeyType key;

        /**
         * The list of values associated with this entry's key. This always
         * contains at least one value.
         */
        ValueType[] values;

        /**
         * Gets a human-readable string representation of this entry.
         * Returns: A string representation of this entry.
         */
        string toString() const {
            import std.conv : to;
            import std.algorithm : map, joiner;
            import std.array : array;

            string keyStr = key.to!string;
            string valuesStr = values
                .map!(v => "\""~v.to!string~"\"")
                .joiner(", ").array.to!string;
            return "\"" ~ keyStr ~ "\": " ~ valuesStr;
        }
    }

    /// The internal, sorted array of entries.
    private Entry[] entries;

    /**
     * Finds the index of the entry with a given key in the internal array.
     * Params:
     *   k = The key to search for.
     * Returns: The index if it was found, or -1 if it doesn't exist.
     */
    private long indexOf(KeyType k) const {
        if (entries.length == 0) return -1;
        if (entries.length == 1) {
            return entries[0].key == k ? 0 : -1;
        }
        static if (Sorted) {
            size_t startIdx = 0;
            size_t endIdx = entries.length - 1;
            while (startIdx <= endIdx) {
                size_t mid = startIdx + (endIdx - startIdx) / 2;
                if (entries[mid].key == k) return mid;
                if (KeySort(entries[mid].key, k)) {
                    startIdx = mid + 1;
                } else {
                    endIdx = mid - 1;
                }
            }
            return -1;
        } else {
            for (size_t idx = 0; idx < entries.length; idx++) {
                if (entries[idx].key == k) return idx;
            }
            return -1;
        }
    }

    /**
     * Attempts to get the entry for a given key. Complexity is O(log(keyCount)).
     * Params:
     *   k = The key to look for.
     * Returns: An optional that may contain the entry that was found.
     */
    private Optional!Entry getEntry(KeyType k) {
        long idx = indexOf(k);
        if (idx == -1) return Optional!Entry.empty();
        return Optional!Entry.of(entries[cast(size_t) idx]);
    }

    /**
     * Gets the number of unique keys in this map.
     * Returns: The number of unique keys in this map.
     */
    size_t length() const {
        return entries.length;
    }

    /**
     * Determines if this map contains a value for the given key.
     * Params:
     *   k = The key to search for.
     * Returns: True if at least one value exists for the given key.
     */
    bool contains(KeyType k) const {
        return indexOf(k) != -1;
    }

    /**
     * Gets a list of all keys in this map, allocated in a new array.
     * Returns: The list of keys in this map.
     */
    KeyType[] keys() const {
        KeyType[] keysArray = new KeyType[this.length()];
        foreach (size_t i, const Entry e; entries) {
            keysArray[i] = e.key;
        }
        return keysArray;
    }

    /**
     * Gets all values associated with a given key, allocated in a new array.
     * Params:
     *   k = The key to get the values of.
     * Returns: The values associated with the given key, or an empty array if
     * no values exist for the key.
     */
    ValueType[] getAll(KeyType k) const {
        long idx = indexOf(k);
        if (idx == -1) return [];
        return entries[cast(size_t) idx].values.dup;
    }

    /**
     * Gets the first value associated with a given key, as per the order in
     * which the values were inserted.
     * Params:
     *   k = The key to get the first value of.
     * Returns: An optional contains the value, if there is at least one value
     * for the given key.
     */
    Optional!ValueType getFirst(KeyType k) const {
        long idx = indexOf(k);
        if (idx == -1) return Optional!ValueType.empty();
        return Optional!ValueType.of(entries[cast(size_t) idx].values[0]);
    }

    /**
     * Adds a single key -> value pair to the map, with time complexity of
     * O(n*log(n)) due to sorting the new entry by its key.
     * Params:
     *   k = The key.
     *   v = The value associated with the key.
     */
    void add(KeyType k, ValueType v) {
        long idx = indexOf(k);
        if (idx == -1) {
            entries ~= Entry(k, [v]);
            static if (Sorted) {
                import std.algorithm.sorting : sort;
                sort!((a, b) => KeySort(a.key, b.key))(entries);
            }
        } else {
            entries[cast(size_t) idx].values ~= v;
        }
    }

    /**
     * Clears this map of all values.
     */
    void clear() {
        entries.length = 0;
    }

    /**
     * Removes a key from the map, thus removing all values associated with
     * that key.
     * Params:
     *   k = The key to remove.
     */
    void remove(KeyType k) {
        long idx = indexOf(k);
        if (idx == -1) return;
        if (entries.length == 1) {
            clear();
            return;
        }
        if (idx + 1 < entries.length) {
            const i = cast(size_t) idx;
            entries[i .. $ - 1] = entries[i + 1 .. $];
        }
        entries.length = entries.length - 1;
    }

    /**
     * Gets this multivalue map as an associative array, where each key is
     * mapped to a list of values.
     * Returns: The associative array.
     */
    ValueType[][KeyType] asAssociativeArray() const {
        ValueType[][KeyType] aa;
        foreach (const Entry entry; entries) {
            aa[entry.key] = entry.values.dup;
        }
        return aa;
    }

    /**
     * Constructs a multivalued map from an associative array.
     * Params:
     *   aa = The associative array to use.
     * Returns: The multivalued map.
     */
    static MultiValueMap fromAssociativeArray(ValueType[][KeyType] aa) {
        MultiValueMap m;
        foreach (KeyType k, ValueType[] values; aa) {
            foreach (ValueType v; values) {
                m.add(k, v);
            }
        }
        return m;
    }

    /**
     * Constructs a multivalued map from an associative array of single values.
     * Params:
     *   aa = The associative array to use.
     * Returns: The multivalued map.
     */
    static MultiValueMap fromAssociativeArray(ValueType[KeyType] aa) {
        MultiValueMap m;
        foreach (KeyType k, ValueType v; aa) {
            m.add(k, v);
        }
        return m;
    }

    /**
     * An efficient builder that can be used to construct a multivalued map
     * with successive `add` calls, which is more efficient than doing so
     * directly due to the builder's deferred sorting.
     */
    static struct Builder {
        import std.array;

        private MultiValueMap m;
        private RefAppender!(Entry[]) entryAppender;

        /**
         * Adds a key -> value pair to the builder's map.
         * Params:
         *   k = The key.
         *   v = The value associated with the key.
         * Returns: A reference to the builder, for method chaining.
         */
        ref Builder add(KeyType k, ValueType v) {
            if (entryAppender.data is null) entryAppender = appender(&m.entries);
            long idx = this.indexOf(k);
            if (idx == -1) {
                entryAppender ~= Entry(k, [v]);
            } else {
                m.entries[cast(size_t) idx].values ~= v;
            }
            return this;
        }

        /**
         * Builds the multivalued map.
         * Returns: The map that was created.
         */
        MultiValueMap build() {
            if (m.entries.length == 0) return m;
            static if (Sorted) {
                import std.algorithm.sorting : sort;
                sort!((a, b) => KeySort(a.key, b.key))(m.entries);
            }
            return m;
        }

        private long indexOf(KeyType k) {
            foreach (i, entry; m.entries) {
                if (entry.key == k) return i;
            }
            return -1;
        }
    }

    // OPERATOR OVERLOADS below here

    /**
     * Implements the empty index operator, which just returns the entire list
     * of entries in this map.
     * Returns: The list of entries in this map.
     */
    inout(Entry)[] opIndex() inout {
        return entries;
    }

    /**
     * Convenience overload to get the first value for a given key. Note: this
     * will throw an exception if no values exist for the given key. To avoid
     * this, use `getFirst` and deal with the missing value yourself.
     * Params:
     *   key = The key to get the value of.
     * Returns: The first value for the given key.
     */
    ValueType opIndex(KeyType key) const {
        import std.conv : to;
        return getFirst(key).orElseThrow("No values exist for key " ~ key.to!string ~ ".");
    }

    /**
     * `opApply` implementation to allow iterating over this map by all pairs
     * of keys and values.
     * Params:
     *   dg = The foreach body that uses each key -> value pair.
     * Returns: The result of the delegate call.
     */
    int opApply(int delegate(const ref KeyType, const ref ValueType) dg) const {
        int result = 0;
        foreach (const Entry entry; entries) {
            foreach (ValueType value; entry.values) {
                result = dg(entry.key, value);
                if (result) break;
            }
        }
        return result;
    }

    /**
     * Implements opBinaryRight for the "in" operator, such that `k in m` will
     * resolve to the list of values for key `k` in the multivalue map `m` if
     * that key exists, or `null` if not.
     *
     * Params:
     *   lhs = The key to use.
     * Returns: A list of values for the given key, or null if no such key exists.
     * ---
     * StringMultiValueMap m;
     * m.add("a", "hello");
     * assert("a" in m);
     * assert(("a" in m) == ["hello"]);
     * assert("b" !in m);
     * assert(("b" in m) is null);
     * ---
     */
    ValueType[] opBinaryRight(string op : "in")(string lhs) {
        Optional!Entry optionalEntry = this.getEntry(lhs);
        if (optionalEntry) {
            Entry entry = optionalEntry.value;
            return entry.values;
        }
        return null;
    }

    /**
     * Converts this map into a human-readable string which lists each key and
     * all of the values for that key.
     * Returns: A string representation of this map.
     */
    string toString() const {
        import std.format : format;
        return format!"%(%s\n%)"(entries);
    }
}

/**
 * A multivalued map of strings, where each string key refers to zero or more
 * string values. All keys are case-sensitive.
 */
alias StringMultiValueMap = MultiValueMap!(string, string);

unittest {
    StringMultiValueMap m;
    m.add("a", "hello");
    assert(m.getFirst("a").orElseThrow == "hello");
    m.add("b", "bye");
    assert(m.getFirst("b").orElseThrow == "bye");
    assert(m.asAssociativeArray == ["a": ["hello"], "b": ["bye"]]);
    assert(m["b"] == "bye");
    m.remove("a");
    assert(!m.contains("a"));
    m.add("b", "hello");
    assert(m.getAll("b") == ["bye", "hello"]);
    m.clear();
    assert(m.length == 0);
    assert(!m.contains("a"));
    assert(!m.contains("b"));

    auto m2 = StringMultiValueMap.fromAssociativeArray(["a": "123", "b": "abc"]);
    assert(m2["a"] == "123");
    assert(m2["b"] == "abc");

    auto m3 = StringMultiValueMap.fromAssociativeArray(["a": [""], "b": [""], "c": ["hello"]]);
    assert(m3.contains("a"));
    assert(m3["a"] == "");
    assert(m3.contains("b"));
    assert(m3["b"] == "");
    assert(m3.contains("c"));
    assert(m3["c"] == "hello");

    // Test that opApply works:
    int n = 0;
    foreach (key, value; m3) {
        n++;
    }
    assert(n == 3);

    // Test opBinaryRight with "in" operator.
    StringMultiValueMap m4;
    m4.add("a", "1");
    assert("a" in m4);
    assert("b" !in m4);
    auto valuesA = "a" in m4;
    assert(valuesA == ["1"]);
    auto valuesB = "b" in m4;
    assert(valuesB is null);

    // Test opIndex with an empty index.
    StringMultiValueMap m5;
    assert(m5[] == []);
    m5.add("a", "123");
    assert(m5[] == [StringMultiValueMap.Entry("a", ["123"])]);

    // test on a const instance
    const(StringMultiValueMap) m6 = m5;
    assert(m6[] == [StringMultiValueMap.Entry("a", ["123"])]);

    // test the builder with multi-values
    StringMultiValueMap.Builder builder;
    builder.add("a", "123");
    builder.add("a", "456");
    assert(builder.build()[] == [StringMultiValueMap.Entry("a", ["123", "456"])], builder.build().toString);
}