const std = @import("std");
const assert = std.debug.assert;
const math = std.math;
const mem = std.mem;
const sort = std.sort;
const testing = std.testing;

const consts = @import("consts.zig").huffman;

const LiteralNode = struct {
    literal: u16,
    freq: u16,
};

// Describes the state of the constructed tree for a given depth.
const LevelInfo = struct {
    // Our level.  for better printing
    level: u32,

    // The frequency of the last node at this level
    last_freq: u32,

    // The frequency of the next character to add to this level
    next_char_freq: u32,

    // The frequency of the next pair (from level below) to add to this level.
    // Only valid if the "needed" value of the next lower level is 0.
    next_pair_freq: u32,

    // The number of chains remaining to generate for this level before moving
    // up to the next level
    needed: u32,
};

// hcode is a huffman code with a bit code and bit length.
pub const HuffCode = struct {
    code: u16 = 0,
    len: u16 = 0,

    // set sets the code and length of an hcode.
    fn set(self: *HuffCode, code: u16, length: u16) void {
        self.len = length;
        self.code = code;
    }
};

pub fn HuffmanEncoder(comptime size: usize) type {
    return struct {
        codes: [size]HuffCode = undefined,
        // Reusable buffer with the longest possible frequency table.
        freq_cache: [consts.max_num_frequencies + 1]LiteralNode = undefined,
        bit_count: [17]u32 = undefined,
        lns: []LiteralNode = undefined, // sorted by literal, stored to avoid repeated allocation in generate
        lfs: []LiteralNode = undefined, // sorted by frequency, stored to avoid repeated allocation in generate

        const Self = @This();

        // Update this Huffman Code object to be the minimum code for the specified frequency count.
        //
        // freq  An array of frequencies, in which frequency[i] gives the frequency of literal i.
        // max_bits  The maximum number of bits to use for any literal.
        pub fn generate(self: *Self, freq: []u16, max_bits: u32) void {
            var list = self.freq_cache[0 .. freq.len + 1];
            // Number of non-zero literals
            var count: u32 = 0;
            // Set list to be the set of all non-zero literals and their frequencies
            for (freq, 0..) |f, i| {
                if (f != 0) {
                    list[count] = LiteralNode{ .literal = @as(u16, @intCast(i)), .freq = f };
                    count += 1;
                } else {
                    list[count] = LiteralNode{ .literal = 0x00, .freq = 0 };
                    self.codes[i].len = 0;
                }
            }
            list[freq.len] = LiteralNode{ .literal = 0x00, .freq = 0 };

            list = list[0..count];
            if (count <= 2) {
                // Handle the small cases here, because they are awkward for the general case code. With
                // two or fewer literals, everything has bit length 1.
                for (list, 0..) |node, i| {
                    // "list" is in order of increasing literal value.
                    self.codes[node.literal].set(@as(u16, @intCast(i)), 1);
                }
                return;
            }
            self.lfs = list;
            mem.sort(LiteralNode, self.lfs, {}, byFreq);

            // Get the number of literals for each bit count
            const bit_count = self.bitCounts(list, max_bits);
            // And do the assignment
            self.assignEncodingAndSize(bit_count, list);
        }

        pub fn bitLength(self: *Self, freq: []u16) u32 {
            var total: u32 = 0;
            for (freq, 0..) |f, i| {
                if (f != 0) {
                    total += @as(u32, @intCast(f)) * @as(u32, @intCast(self.codes[i].len));
                }
            }
            return total;
        }

        // Return the number of literals assigned to each bit size in the Huffman encoding
        //
        // This method is only called when list.len >= 3
        // The cases of 0, 1, and 2 literals are handled by special case code.
        //
        // list: An array of the literals with non-zero frequencies
        // and their associated frequencies. The array is in order of increasing
        // frequency, and has as its last element a special element with frequency
        // std.math.maxInt(i32)
        //
        // max_bits: The maximum number of bits that should be used to encode any literal.
        // Must be less than 16.
        //
        // Returns an integer array in which array[i] indicates the number of literals
        // that should be encoded in i bits.
        fn bitCounts(self: *Self, list: []LiteralNode, max_bits_to_use: usize) []u32 {
            var max_bits = max_bits_to_use;
            const n = list.len;
            const max_bits_limit = 16;

            assert(max_bits < max_bits_limit);

            // The tree can't have greater depth than n - 1, no matter what. This
            // saves a little bit of work in some small cases
            max_bits = @min(max_bits, n - 1);

            // Create information about each of the levels.
            // A bogus "Level 0" whose sole purpose is so that
            // level1.prev.needed == 0.  This makes level1.next_pair_freq
            // be a legitimate value that never gets chosen.
            var levels: [max_bits_limit]LevelInfo = mem.zeroes([max_bits_limit]LevelInfo);
            // leaf_counts[i] counts the number of literals at the left
            // of ancestors of the rightmost node at level i.
            // leaf_counts[i][j] is the number of literals at the left
            // of the level j ancestor.
            var leaf_counts: [max_bits_limit][max_bits_limit]u32 = mem.zeroes([max_bits_limit][max_bits_limit]u32);

            {
                var level = @as(u32, 1);
                while (level <= max_bits) : (level += 1) {
                    // For every level, the first two items are the first two characters.
                    // We initialize the levels as if we had already figured this out.
                    levels[level] = LevelInfo{
                        .level = level,
                        .last_freq = list[1].freq,
                        .next_char_freq = list[2].freq,
                        .next_pair_freq = list[0].freq + list[1].freq,
                        .needed = 0,
                    };
                    leaf_counts[level][level] = 2;
                    if (level == 1) {
                        levels[level].next_pair_freq = math.maxInt(i32);
                    }
                }
            }

            // We need a total of 2*n - 2 items at top level and have already generated 2.
            levels[max_bits].needed = 2 * @as(u32, @intCast(n)) - 4;

            {
                var level = max_bits;
                while (true) {
                    var l = &levels[level];
                    if (l.next_pair_freq == math.maxInt(i32) and l.next_char_freq == math.maxInt(i32)) {
                        // We've run out of both leaves and pairs.
                        // End all calculations for this level.
                        // To make sure we never come back to this level or any lower level,
                        // set next_pair_freq impossibly large.
                        l.needed = 0;
                        levels[level + 1].next_pair_freq = math.maxInt(i32);
                        level += 1;
                        continue;
                    }

                    const prev_freq = l.last_freq;
                    if (l.next_char_freq < l.next_pair_freq) {
                        // The next item on this row is a leaf node.
                        const next = leaf_counts[level][level] + 1;
                        l.last_freq = l.next_char_freq;
                        // Lower leaf_counts are the same of the previous node.
                        leaf_counts[level][level] = next;
                        if (next >= list.len) {
                            l.next_char_freq = maxNode().freq;
                        } else {
                            l.next_char_freq = list[next].freq;
                        }
                    } else {
                        // The next item on this row is a pair from the previous row.
                        // next_pair_freq isn't valid until we generate two
                        // more values in the level below
                        l.last_freq = l.next_pair_freq;
                        // Take leaf counts from the lower level, except counts[level] remains the same.
                        @memcpy(leaf_counts[level][0..level], leaf_counts[level - 1][0..level]);
                        levels[l.level - 1].needed = 2;
                    }

                    l.needed -= 1;
                    if (l.needed == 0) {
                        // We've done everything we need to do for this level.
                        // Continue calculating one level up. Fill in next_pair_freq
                        // of that level with the sum of the two nodes we've just calculated on
                        // this level.
                        if (l.level == max_bits) {
                            // All done!
                            break;
                        }
                        levels[l.level + 1].next_pair_freq = prev_freq + l.last_freq;
                        level += 1;
                    } else {
                        // If we stole from below, move down temporarily to replenish it.
                        while (levels[level - 1].needed > 0) {
                            level -= 1;
                            if (level == 0) {
                                break;
                            }
                        }
                    }
                }
            }

            // Somethings is wrong if at the end, the top level is null or hasn't used
            // all of the leaves.
            assert(leaf_counts[max_bits][max_bits] == n);

            var bit_count = self.bit_count[0 .. max_bits + 1];
            var bits: u32 = 1;
            const counts = &leaf_counts[max_bits];
            {
                var level = max_bits;
                while (level > 0) : (level -= 1) {
                    // counts[level] gives the number of literals requiring at least "bits"
                    // bits to encode.
                    bit_count[bits] = counts[level] - counts[level - 1];
                    bits += 1;
                    if (level == 0) {
                        break;
                    }
                }
            }
            return bit_count;
        }

        // Look at the leaves and assign them a bit count and an encoding as specified
        // in RFC 1951 3.2.2
        fn assignEncodingAndSize(self: *Self, bit_count: []u32, list_arg: []LiteralNode) void {
            var code = @as(u16, 0);
            var list = list_arg;

            for (bit_count, 0..) |bits, n| {
                code <<= 1;
                if (n == 0 or bits == 0) {
                    continue;
                }
                // The literals list[list.len-bits] .. list[list.len-bits]
                // are encoded using "bits" bits, and get the values
                // code, code + 1, ....  The code values are
                // assigned in literal order (not frequency order).
                const chunk = list[list.len - @as(u32, @intCast(bits)) ..];

                self.lns = chunk;
                mem.sort(LiteralNode, self.lns, {}, byLiteral);

                for (chunk) |node| {
                    self.codes[node.literal] = HuffCode{
                        .code = bitReverse(u16, code, @as(u5, @intCast(n))),
                        .len = @as(u16, @intCast(n)),
                    };
                    code += 1;
                }
                list = list[0 .. list.len - @as(u32, @intCast(bits))];
            }
        }
    };
}

fn maxNode() LiteralNode {
    return LiteralNode{
        .literal = math.maxInt(u16),
        .freq = math.maxInt(u16),
    };
}

pub fn huffmanEncoder(comptime size: u32) HuffmanEncoder(size) {
    return .{};
}

pub const LiteralEncoder = HuffmanEncoder(consts.max_num_frequencies);
pub const DistanceEncoder = HuffmanEncoder(consts.distance_code_count);
pub const CodegenEncoder = HuffmanEncoder(19);

// Generates a HuffmanCode corresponding to the fixed literal table
pub fn fixedLiteralEncoder() LiteralEncoder {
    var h: LiteralEncoder = undefined;
    var ch: u16 = 0;

    while (ch < consts.max_num_frequencies) : (ch += 1) {
        var bits: u16 = undefined;
        var size: u16 = undefined;
        switch (ch) {
            0...143 => {
                // size 8, 000110000  .. 10111111
                bits = ch + 48;
                size = 8;
            },
            144...255 => {
                // size 9, 110010000 .. 111111111
                bits = ch + 400 - 144;
                size = 9;
            },
            256...279 => {
                // size 7, 0000000 .. 0010111
                bits = ch - 256;
                size = 7;
            },
            else => {
                // size 8, 11000000 .. 11000111
                bits = ch + 192 - 280;
                size = 8;
            },
        }
        h.codes[ch] = HuffCode{ .code = bitReverse(u16, bits, @as(u5, @intCast(size))), .len = size };
    }
    return h;
}

pub fn fixedDistanceEncoder() DistanceEncoder {
    var h: DistanceEncoder = undefined;
    for (h.codes, 0..) |_, ch| {
        h.codes[ch] = HuffCode{ .code = bitReverse(u16, @as(u16, @intCast(ch)), 5), .len = 5 };
    }
    return h;
}

pub fn huffmanDistanceEncoder() DistanceEncoder {
    var distance_freq = [1]u16{0} ** consts.distance_code_count;
    distance_freq[0] = 1;
    // huff_distance is a static distance encoder used for huffman only encoding.
    // It can be reused since we will not be encoding distance values.
    var h: DistanceEncoder = .{};
    h.generate(distance_freq[0..], 15);
    return h;
}

fn byLiteral(context: void, a: LiteralNode, b: LiteralNode) bool {
    _ = context;
    return a.literal < b.literal;
}

fn byFreq(context: void, a: LiteralNode, b: LiteralNode) bool {
    _ = context;
    if (a.freq == b.freq) {
        return a.literal < b.literal;
    }
    return a.freq < b.freq;
}

test "generate a Huffman code from an array of frequencies" {
    var freqs: [19]u16 = [_]u16{
        8, // 0
        1, // 1
        1, // 2
        2, // 3
        5, // 4
        10, // 5
        9, // 6
        1, // 7
        0, // 8
        0, // 9
        0, // 10
        0, // 11
        0, // 12
        0, // 13
        0, // 14
        0, // 15
        1, // 16
        3, // 17
        5, // 18
    };

    var enc = huffmanEncoder(19);
    enc.generate(freqs[0..], 7);

    try testing.expectEqual(@as(u32, 141), enc.bitLength(freqs[0..]));

    try testing.expectEqual(@as(usize, 3), enc.codes[0].len);
    try testing.expectEqual(@as(usize, 6), enc.codes[1].len);
    try testing.expectEqual(@as(usize, 6), enc.codes[2].len);
    try testing.expectEqual(@as(usize, 5), enc.codes[3].len);
    try testing.expectEqual(@as(usize, 3), enc.codes[4].len);
    try testing.expectEqual(@as(usize, 2), enc.codes[5].len);
    try testing.expectEqual(@as(usize, 2), enc.codes[6].len);
    try testing.expectEqual(@as(usize, 6), enc.codes[7].len);
    try testing.expectEqual(@as(usize, 0), enc.codes[8].len);
    try testing.expectEqual(@as(usize, 0), enc.codes[9].len);
    try testing.expectEqual(@as(usize, 0), enc.codes[10].len);
    try testing.expectEqual(@as(usize, 0), enc.codes[11].len);
    try testing.expectEqual(@as(usize, 0), enc.codes[12].len);
    try testing.expectEqual(@as(usize, 0), enc.codes[13].len);
    try testing.expectEqual(@as(usize, 0), enc.codes[14].len);
    try testing.expectEqual(@as(usize, 0), enc.codes[15].len);
    try testing.expectEqual(@as(usize, 6), enc.codes[16].len);
    try testing.expectEqual(@as(usize, 5), enc.codes[17].len);
    try testing.expectEqual(@as(usize, 3), enc.codes[18].len);

    try testing.expectEqual(@as(u16, 0x0), enc.codes[5].code);
    try testing.expectEqual(@as(u16, 0x2), enc.codes[6].code);
    try testing.expectEqual(@as(u16, 0x1), enc.codes[0].code);
    try testing.expectEqual(@as(u16, 0x5), enc.codes[4].code);
    try testing.expectEqual(@as(u16, 0x3), enc.codes[18].code);
    try testing.expectEqual(@as(u16, 0x7), enc.codes[3].code);
    try testing.expectEqual(@as(u16, 0x17), enc.codes[17].code);
    try testing.expectEqual(@as(u16, 0x0f), enc.codes[1].code);
    try testing.expectEqual(@as(u16, 0x2f), enc.codes[2].code);
    try testing.expectEqual(@as(u16, 0x1f), enc.codes[7].code);
    try testing.expectEqual(@as(u16, 0x3f), enc.codes[16].code);
}

test "generate a Huffman code for the fixed literal table specific to Deflate" {
    const enc = fixedLiteralEncoder();
    for (enc.codes) |c| {
        switch (c.len) {
            7 => {
                const v = @bitReverse(@as(u7, @intCast(c.code)));
                try testing.expect(v <= 0b0010111);
            },
            8 => {
                const v = @bitReverse(@as(u8, @intCast(c.code)));
                try testing.expect((v >= 0b000110000 and v <= 0b10111111) or
                    (v >= 0b11000000 and v <= 11000111));
            },
            9 => {
                const v = @bitReverse(@as(u9, @intCast(c.code)));
                try testing.expect(v >= 0b110010000 and v <= 0b111111111);
            },
            else => unreachable,
        }
    }
}

test "generate a Huffman code for the 30 possible relative distances (LZ77 distances) of Deflate" {
    const enc = fixedDistanceEncoder();
    for (enc.codes) |c| {
        const v = @bitReverse(@as(u5, @intCast(c.code)));
        try testing.expect(v <= 29);
        try testing.expect(c.len == 5);
    }
}

// Reverse bit-by-bit a N-bit code.
fn bitReverse(comptime T: type, value: T, n: usize) T {
    const r = @bitReverse(value);
    return r >> @as(math.Log2Int(T), @intCast(@typeInfo(T).int.bits - n));
}

test bitReverse {
    const ReverseBitsTest = struct {
        in: u16,
        bit_count: u5,
        out: u16,
    };

    const reverse_bits_tests = [_]ReverseBitsTest{
        .{ .in = 1, .bit_count = 1, .out = 1 },
        .{ .in = 1, .bit_count = 2, .out = 2 },
        .{ .in = 1, .bit_count = 3, .out = 4 },
        .{ .in = 1, .bit_count = 4, .out = 8 },
        .{ .in = 1, .bit_count = 5, .out = 16 },
        .{ .in = 17, .bit_count = 5, .out = 17 },
        .{ .in = 257, .bit_count = 9, .out = 257 },
        .{ .in = 29, .bit_count = 5, .out = 23 },
    };

    for (reverse_bits_tests) |h| {
        const v = bitReverse(u16, h.in, h.bit_count);
        try std.testing.expectEqual(h.out, v);
    }
}

// FIXME: This test does not work in Zig 0.15.0, and I don't anticipate this
// code being here for long.
//
// test "fixedLiteralEncoder codes" {
//     var al = std.ArrayList(u8).init(testing.allocator);
//     defer al.deinit();
//     var bw = std.io.bitWriter(.little, al.writer());
//
//     const f = fixedLiteralEncoder();
//     for (f.codes) |c| {
//         try bw.writeBits(c.code, c.len);
//     }
//     try testing.expectEqualSlices(u8, &fixed_codes, al.items);
// }

pub const fixed_codes = [_]u8{
    0b00001100, 0b10001100, 0b01001100, 0b11001100, 0b00101100, 0b10101100, 0b01101100, 0b11101100,
    0b00011100, 0b10011100, 0b01011100, 0b11011100, 0b00111100, 0b10111100, 0b01111100, 0b11111100,
    0b00000010, 0b10000010, 0b01000010, 0b11000010, 0b00100010, 0b10100010, 0b01100010, 0b11100010,
    0b00010010, 0b10010010, 0b01010010, 0b11010010, 0b00110010, 0b10110010, 0b01110010, 0b11110010,
    0b00001010, 0b10001010, 0b01001010, 0b11001010, 0b00101010, 0b10101010, 0b01101010, 0b11101010,
    0b00011010, 0b10011010, 0b01011010, 0b11011010, 0b00111010, 0b10111010, 0b01111010, 0b11111010,
    0b00000110, 0b10000110, 0b01000110, 0b11000110, 0b00100110, 0b10100110, 0b01100110, 0b11100110,
    0b00010110, 0b10010110, 0b01010110, 0b11010110, 0b00110110, 0b10110110, 0b01110110, 0b11110110,
    0b00001110, 0b10001110, 0b01001110, 0b11001110, 0b00101110, 0b10101110, 0b01101110, 0b11101110,
    0b00011110, 0b10011110, 0b01011110, 0b11011110, 0b00111110, 0b10111110, 0b01111110, 0b11111110,
    0b00000001, 0b10000001, 0b01000001, 0b11000001, 0b00100001, 0b10100001, 0b01100001, 0b11100001,
    0b00010001, 0b10010001, 0b01010001, 0b11010001, 0b00110001, 0b10110001, 0b01110001, 0b11110001,
    0b00001001, 0b10001001, 0b01001001, 0b11001001, 0b00101001, 0b10101001, 0b01101001, 0b11101001,
    0b00011001, 0b10011001, 0b01011001, 0b11011001, 0b00111001, 0b10111001, 0b01111001, 0b11111001,
    0b00000101, 0b10000101, 0b01000101, 0b11000101, 0b00100101, 0b10100101, 0b01100101, 0b11100101,
    0b00010101, 0b10010101, 0b01010101, 0b11010101, 0b00110101, 0b10110101, 0b01110101, 0b11110101,
    0b00001101, 0b10001101, 0b01001101, 0b11001101, 0b00101101, 0b10101101, 0b01101101, 0b11101101,
    0b00011101, 0b10011101, 0b01011101, 0b11011101, 0b00111101, 0b10111101, 0b01111101, 0b11111101,
    0b00010011, 0b00100110, 0b01001110, 0b10011010, 0b00111100, 0b01100101, 0b11101010, 0b10110100,
    0b11101001, 0b00110011, 0b01100110, 0b11001110, 0b10011010, 0b00111101, 0b01100111, 0b11101110,
    0b10111100, 0b11111001, 0b00001011, 0b00010110, 0b00101110, 0b01011010, 0b10111100, 0b01100100,
    0b11101001, 0b10110010, 0b11100101, 0b00101011, 0b01010110, 0b10101110, 0b01011010, 0b10111101,
    0b01100110, 0b11101101, 0b10111010, 0b11110101, 0b00011011, 0b00110110, 0b01101110, 0b11011010,
    0b10111100, 0b01100101, 0b11101011, 0b10110110, 0b11101101, 0b00111011, 0b01110110, 0b11101110,
    0b11011010, 0b10111101, 0b01100111, 0b11101111, 0b10111110, 0b11111101, 0b00000111, 0b00001110,
    0b00011110, 0b00111010, 0b01111100, 0b11100100, 0b11101000, 0b10110001, 0b11100011, 0b00100111,
    0b01001110, 0b10011110, 0b00111010, 0b01111101, 0b11100110, 0b11101100, 0b10111001, 0b11110011,
    0b00010111, 0b00101110, 0b01011110, 0b10111010, 0b01111100, 0b11100101, 0b11101010, 0b10110101,
    0b11101011, 0b00110111, 0b01101110, 0b11011110, 0b10111010, 0b01111101, 0b11100111, 0b11101110,
    0b10111101, 0b11111011, 0b00001111, 0b00011110, 0b00111110, 0b01111010, 0b11111100, 0b11100100,
    0b11101001, 0b10110011, 0b11100111, 0b00101111, 0b01011110, 0b10111110, 0b01111010, 0b11111101,
    0b11100110, 0b11101101, 0b10111011, 0b11110111, 0b00011111, 0b00111110, 0b01111110, 0b11111010,
    0b11111100, 0b11100101, 0b11101011, 0b10110111, 0b11101111, 0b00111111, 0b01111110, 0b11111110,
    0b11111010, 0b11111101, 0b11100111, 0b11101111, 0b10111111, 0b11111111, 0b00000000, 0b00100000,
    0b00001000, 0b00001100, 0b10000001, 0b11000010, 0b11100000, 0b00001000, 0b00100100, 0b00001010,
    0b10001101, 0b11000001, 0b11100010, 0b11110000, 0b00000100, 0b00100010, 0b10001001, 0b01001100,
    0b10100001, 0b11010010, 0b11101000, 0b00000011, 0b10000011, 0b01000011, 0b11000011, 0b00100011,
    0b10100011,
};
