// Based on Go stdlib implementation

const std = @import("../std.zig");
const mem = std.mem;
const debug = std.debug;

/// Counter mode.
///
/// This mode creates a key stream by encrypting an incrementing counter using a block cipher, and adding it to the source material.
///
/// Important: the counter mode doesn't provide authenticated encryption: the ciphertext can be trivially modified without this being detected.
/// As a result, applications should generally never use it directly, but only in a construction that includes a MAC.
pub fn ctr(comptime BlockCipher: anytype, block_cipher: BlockCipher, dst: []u8, src: []const u8, iv: [BlockCipher.block_length]u8, endian: std.builtin.Endian) void {
    ctrSlice(BlockCipher, block_cipher, dst, src, iv, endian, 0, BlockCipher.block_length);
}

/// Counter mode with configurable counter position and size.
///
/// This extended version allows specifying where the counter is located within the IV block
/// and how many bytes it occupies. This is useful for modes like AES-GCM-SIV which use a
/// 32-bit counter at the beginning of the block.
///
/// @param counter_offset: Byte offset where the counter starts
/// @param counter_size: Size of the counter in bytes
pub fn ctrSlice(
    comptime BlockCipher: anytype,
    block_cipher: BlockCipher,
    dst: []u8,
    src: []const u8,
    iv: [BlockCipher.block_length]u8,
    endian: std.builtin.Endian,
    comptime counter_offset: usize,
    comptime counter_size: usize,
) void {
    debug.assert(dst.len >= src.len);
    const block_length = BlockCipher.block_length;
    debug.assert(counter_offset + counter_size <= block_length);
    debug.assert(counter_size > 0 and counter_size <= block_length);

    var counterBlock = iv;
    var i: usize = 0;

    const CounterInt = @Int(.unsigned, counter_size * 8);

    const parallel_count = BlockCipher.block.parallel.optimal_parallel_blocks;
    const wide_block_length = parallel_count * block_length;
    var cnt_val = mem.readInt(CounterInt, counterBlock[counter_offset..][0..counter_size], endian);
    if (src.len >= wide_block_length) {
        var counters: [parallel_count * block_length]u8 = undefined;
        inline for (0..parallel_count) |j| {
            counters[j * block_length ..][0..block_length].* = iv;
        }
        while (i + wide_block_length <= src.len) : (i += wide_block_length) {
            comptime var j = 0;
            inline while (j < parallel_count) : (j += 1) {
                mem.writeInt(CounterInt, counters[j * block_length + counter_offset ..][0..counter_size], cnt_val +% j, endian);
            }
            cnt_val += parallel_count;
            block_cipher.xorWide(parallel_count, dst[i .. i + wide_block_length][0..wide_block_length], src[i .. i + wide_block_length][0..wide_block_length], counters);
        }
        mem.writeInt(CounterInt, counterBlock[counter_offset..][0..counter_size], cnt_val, endian);
    }
    while (i + block_length <= src.len) : (i += block_length) {
        block_cipher.xor(dst[i .. i + block_length][0..block_length], src[i .. i + block_length][0..block_length], counterBlock);
        cnt_val +%= 1;
        mem.writeInt(CounterInt, counterBlock[counter_offset..][0..counter_size], cnt_val, endian);
    }
    if (i < src.len) {
        var pad: [block_length]u8 = @splat(0);
        const src_slice = src[i..];
        @memcpy(pad[0..src_slice.len], src_slice);
        block_cipher.xor(&pad, &pad, counterBlock);
        const pad_slice = pad[0 .. src.len - i];
        @memcpy(dst[i..][0..pad_slice.len], pad_slice);
    }
}

test "ctr mode" {
    const testing = std.testing;
    const aes = std.crypto.core.aes;

    // Test key and IV from NIST SP 800-38A
    const key = [_]u8{ 0x2b, 0x7e, 0x15, 0x16, 0x28, 0xae, 0xd2, 0xa6, 0xab, 0xf7, 0x15, 0x88, 0x09, 0xcf, 0x4f, 0x3c };
    const iv = [_]u8{ 0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8, 0xf9, 0xfa, 0xfb, 0xfc, 0xfd, 0xfe, 0xff };
    const ctx = aes.Aes128.initEnc(key);

    // Test 1: Empty input
    {
        const in = [_]u8{};
        const expected = [_]u8{};
        var out: [0]u8 = undefined;
        ctr(aes.AesEncryptCtx(aes.Aes128), ctx, out[0..], in[0..], iv, std.builtin.Endian.big);
        try testing.expectEqualSlices(u8, expected[0..], out[0..]);
    }

    // Test 2: Single byte
    {
        const in = [_]u8{0x6b};
        const expected = [_]u8{0x87};
        var out: [1]u8 = undefined;
        ctr(aes.AesEncryptCtx(aes.Aes128), ctx, out[0..], in[0..], iv, std.builtin.Endian.big);
        try testing.expectEqualSlices(u8, expected[0..], out[0..]);
    }

    // Test 3: Less than one block (15 bytes)
    {
        const in = [_]u8{ 0x6b, 0xc1, 0xbe, 0xe2, 0x2e, 0x40, 0x9f, 0x96, 0xe9, 0x3d, 0x7e, 0x11, 0x73, 0x93, 0x17 };
        const expected = [_]u8{ 0x87, 0x4d, 0x61, 0x91, 0xb6, 0x20, 0xe3, 0x26, 0x1b, 0xef, 0x68, 0x64, 0x99, 0x0d, 0xb6 };
        var out: [15]u8 = undefined;
        ctr(aes.AesEncryptCtx(aes.Aes128), ctx, out[0..], in[0..], iv, std.builtin.Endian.big);
        try testing.expectEqualSlices(u8, expected[0..], out[0..]);
    }

    // Test 4: Exactly one block (16 bytes)
    {
        const in = [_]u8{ 0x6b, 0xc1, 0xbe, 0xe2, 0x2e, 0x40, 0x9f, 0x96, 0xe9, 0x3d, 0x7e, 0x11, 0x73, 0x93, 0x17, 0x2a };
        const expected = [_]u8{ 0x87, 0x4d, 0x61, 0x91, 0xb6, 0x20, 0xe3, 0x26, 0x1b, 0xef, 0x68, 0x64, 0x99, 0x0d, 0xb6, 0xce };
        var out: [16]u8 = undefined;
        ctr(aes.AesEncryptCtx(aes.Aes128), ctx, out[0..], in[0..], iv, std.builtin.Endian.big);
        try testing.expectEqualSlices(u8, expected[0..], out[0..]);
    }

    // Test 5: One block plus one byte (17 bytes)
    {
        const in = [_]u8{ 0x6b, 0xc1, 0xbe, 0xe2, 0x2e, 0x40, 0x9f, 0x96, 0xe9, 0x3d, 0x7e, 0x11, 0x73, 0x93, 0x17, 0x2a, 0xae };
        const expected = [_]u8{ 0x87, 0x4d, 0x61, 0x91, 0xb6, 0x20, 0xe3, 0x26, 0x1b, 0xef, 0x68, 0x64, 0x99, 0x0d, 0xb6, 0xce, 0x98 };
        var out: [17]u8 = undefined;
        ctr(aes.AesEncryptCtx(aes.Aes128), ctx, out[0..], in[0..], iv, std.builtin.Endian.big);
        try testing.expectEqualSlices(u8, expected[0..], out[0..]);
    }

    // Test 6: Exactly two blocks (32 bytes)
    {
        const in = [_]u8{
            0x6b, 0xc1, 0xbe, 0xe2, 0x2e, 0x40, 0x9f, 0x96, 0xe9, 0x3d, 0x7e, 0x11, 0x73, 0x93, 0x17, 0x2a,
            0xae, 0x2d, 0x8a, 0x57, 0x1e, 0x03, 0xac, 0x9c, 0x9e, 0xb7, 0x6f, 0xac, 0x45, 0xaf, 0x8e, 0x51,
        };
        const expected = [_]u8{
            0x87, 0x4d, 0x61, 0x91, 0xb6, 0x20, 0xe3, 0x26, 0x1b, 0xef, 0x68, 0x64, 0x99, 0x0d, 0xb6, 0xce,
            0x98, 0x06, 0xf6, 0x6b, 0x79, 0x70, 0xfd, 0xff, 0x86, 0x17, 0x18, 0x7b, 0xb9, 0xff, 0xfd, 0xff,
        };
        var out: [32]u8 = undefined;
        ctr(aes.AesEncryptCtx(aes.Aes128), ctx, out[0..], in[0..], iv, std.builtin.Endian.big);
        try testing.expectEqualSlices(u8, expected[0..], out[0..]);
    }

    // Test 7: Two blocks plus 5 bytes (37 bytes)
    {
        const in = [_]u8{
            0x6b, 0xc1, 0xbe, 0xe2, 0x2e, 0x40, 0x9f, 0x96, 0xe9, 0x3d, 0x7e, 0x11, 0x73, 0x93, 0x17, 0x2a,
            0xae, 0x2d, 0x8a, 0x57, 0x1e, 0x03, 0xac, 0x9c, 0x9e, 0xb7, 0x6f, 0xac, 0x45, 0xaf, 0x8e, 0x51,
            0x30, 0xc8, 0x1c, 0x46, 0xa3,
        };
        const expected = [_]u8{
            0x87, 0x4d, 0x61, 0x91, 0xb6, 0x20, 0xe3, 0x26, 0x1b, 0xef, 0x68, 0x64, 0x99, 0x0d, 0xb6, 0xce,
            0x98, 0x06, 0xf6, 0x6b, 0x79, 0x70, 0xfd, 0xff, 0x86, 0x17, 0x18, 0x7b, 0xb9, 0xff, 0xfd, 0xff,
            0x5a, 0xe4, 0xdf, 0x3e, 0xdb,
        };
        var out: [37]u8 = undefined;
        ctr(aes.AesEncryptCtx(aes.Aes128), ctx, out[0..], in[0..], iv, std.builtin.Endian.big);
        try testing.expectEqualSlices(u8, expected[0..], out[0..]);
    }

    // Test 8: Four blocks (64 bytes) - NIST test vector
    {
        const in = [_]u8{
            0x6b, 0xc1, 0xbe, 0xe2, 0x2e, 0x40, 0x9f, 0x96, 0xe9, 0x3d, 0x7e, 0x11, 0x73, 0x93, 0x17, 0x2a,
            0xae, 0x2d, 0x8a, 0x57, 0x1e, 0x03, 0xac, 0x9c, 0x9e, 0xb7, 0x6f, 0xac, 0x45, 0xaf, 0x8e, 0x51,
            0x30, 0xc8, 0x1c, 0x46, 0xa3, 0x5c, 0xe4, 0x11, 0xe5, 0xfb, 0xc1, 0x19, 0x1a, 0x0a, 0x52, 0xef,
            0xf6, 0x9f, 0x24, 0x45, 0xdf, 0x4f, 0x9b, 0x17, 0xad, 0x2b, 0x41, 0x7b, 0xe6, 0x6c, 0x37, 0x10,
        };
        const expected = [_]u8{
            0x87, 0x4d, 0x61, 0x91, 0xb6, 0x20, 0xe3, 0x26, 0x1b, 0xef, 0x68, 0x64, 0x99, 0x0d, 0xb6, 0xce,
            0x98, 0x06, 0xf6, 0x6b, 0x79, 0x70, 0xfd, 0xff, 0x86, 0x17, 0x18, 0x7b, 0xb9, 0xff, 0xfd, 0xff,
            0x5a, 0xe4, 0xdf, 0x3e, 0xdb, 0xd5, 0xd3, 0x5e, 0x5b, 0x4f, 0x09, 0x02, 0x0d, 0xb0, 0x3e, 0xab,
            0x1e, 0x03, 0x1d, 0xda, 0x2f, 0xbe, 0x03, 0xd1, 0x79, 0x21, 0x70, 0xa0, 0xf3, 0x00, 0x9c, 0xee,
        };
        var out: [64]u8 = undefined;
        ctr(aes.AesEncryptCtx(aes.Aes128), ctx, out[0..], in[0..], iv, std.builtin.Endian.big);
        try testing.expectEqualSlices(u8, expected[0..], out[0..]);
    }

    // Test 9: Large input (> 2*block_length, 100 bytes)
    {
        // Create a 100-byte input by extending with zeros
        var in: [100]u8 = @splat(0);
        @memcpy(in[0..64], &[_]u8{
            0x6b, 0xc1, 0xbe, 0xe2, 0x2e, 0x40, 0x9f, 0x96, 0xe9, 0x3d, 0x7e, 0x11, 0x73, 0x93, 0x17, 0x2a,
            0xae, 0x2d, 0x8a, 0x57, 0x1e, 0x03, 0xac, 0x9c, 0x9e, 0xb7, 0x6f, 0xac, 0x45, 0xaf, 0x8e, 0x51,
            0x30, 0xc8, 0x1c, 0x46, 0xa3, 0x5c, 0xe4, 0x11, 0xe5, 0xfb, 0xc1, 0x19, 0x1a, 0x0a, 0x52, 0xef,
            0xf6, 0x9f, 0x24, 0x45, 0xdf, 0x4f, 0x9b, 0x17, 0xad, 0x2b, 0x41, 0x7b, 0xe6, 0x6c, 0x37, 0x10,
        });

        // Expected output: first 64 bytes from NIST, then CTR continues with zeros
        var expected: [100]u8 = undefined;
        @memcpy(expected[0..64], &[_]u8{
            0x87, 0x4d, 0x61, 0x91, 0xb6, 0x20, 0xe3, 0x26, 0x1b, 0xef, 0x68, 0x64, 0x99, 0x0d, 0xb6, 0xce,
            0x98, 0x06, 0xf6, 0x6b, 0x79, 0x70, 0xfd, 0xff, 0x86, 0x17, 0x18, 0x7b, 0xb9, 0xff, 0xfd, 0xff,
            0x5a, 0xe4, 0xdf, 0x3e, 0xdb, 0xd5, 0xd3, 0x5e, 0x5b, 0x4f, 0x09, 0x02, 0x0d, 0xb0, 0x3e, 0xab,
            0x1e, 0x03, 0x1d, 0xda, 0x2f, 0xbe, 0x03, 0xd1, 0x79, 0x21, 0x70, 0xa0, 0xf3, 0x00, 0x9c, 0xee,
        });
        // Compute the rest with zeros XORed with keystream
        @memcpy(expected[64..], &[_]u8{
            0xb0, 0x0d, 0x47, 0xf8, 0x14, 0x8a, 0x91, 0x0e, 0xf0, 0x68, 0x30, 0x97, 0x90, 0x4b, 0xa5, 0x02,
            0x58, 0x99, 0x44, 0x5a, 0x4d, 0xe1, 0x01, 0xf5, 0x13, 0xca, 0xd1, 0x98, 0x7d, 0x89, 0xe9, 0x1b,
            0x3b, 0xd9, 0xac, 0x79,
        });

        var out: [100]u8 = undefined;
        ctr(aes.AesEncryptCtx(aes.Aes128), ctx, out[0..], in[0..], iv, std.builtin.Endian.big);
        try testing.expectEqualSlices(u8, expected[0..], out[0..]);
    }

    // Test 10: Test with different endianness (little-endian counter)
    {
        const le_iv = [_]u8{ 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
        const in = [_]u8{ 0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xaa, 0xbb, 0xcc, 0xdd, 0xee, 0xff };

        // We'll compute the expected value from the actual encryption
        var out: [16]u8 = undefined;
        ctr(aes.AesEncryptCtx(aes.Aes128), ctx, out[0..], in[0..], le_iv, std.builtin.Endian.little);

        // The actual output for this test with little-endian counter=1
        const expected = [_]u8{ 0x7e, 0x48, 0x15, 0xa8, 0x16, 0x66, 0xf0, 0xea, 0xad, 0x3c, 0x07, 0x97, 0x2f, 0xe8, 0x25, 0xc1 };
        try testing.expectEqualSlices(u8, expected[0..], out[0..]);
    }
}
