// The source code refers to the go standard library, which will be combined with AES in the future.
// Use of this source code is governed by an MIT license
// that can be found in the LICENSE file.
//
// Counter (CTR) mode.
//
// CTR converts a block cipher into a stream cipher by
// repeatedly encrypting an incrementing counter and
// xoring the resulting stream of data with the input.
//
// See NIST SP 800-38A, pp 13-15
module cipher

import crypto.internal.subtle

struct Ctr {
mut:
    b        Block
    next     []u8
    out      []u8
    out_used int
}

// free the resources taken by the Ctr `c`
@[unsafe]
pub fn (mut x Ctr) free() {
    $if prealloc {
        return
    }
    unsafe {
        // x.b.free() TODO add?
        x.out.free()
        x.next.free()
    }
}

// new_ctr returns a Ctr which encrypts/decrypts using the given Block in
// counter mode. The length of iv must be the same as the Block's block size.
pub fn new_ctr(b Block, iv []u8) Ctr {
    block_size := b.block_size
    if iv.len != block_size {
        panic('cipher.new_cfb: IV length must be equal block size')
    }
    return Ctr{
        b:        b
        out:      []u8{len: b.block_size}
        next:     iv.clone()
        out_used: block_size
    }
}

// xor_key_stream xors each byte in the given slice with a byte from the key stream.
pub fn (mut x Ctr) xor_key_stream(mut dst []u8, src []u8) {
    unsafe {
        mut local_dst := *dst
        mut local_src := src
        if local_dst.len < local_src.len {
            panic('crypto.cipher.xor_key_stream: output smaller than input')
        }

        if subtle.inexact_overlap(local_dst[..local_src.len], local_src) {
            panic('crypto.cipher.xor_key_stream: invalid buffer overlap')
        }

        for local_src.len > 0 {
            if x.out_used == x.out.len {
                x.b.encrypt(mut x.out, x.next)
                x.out_used = 0
                // increment counter
                for i := x.next.len - 1; i >= 0; i-- {
                    x.next[i]++
                    if x.next[i] != 0 {
                        break
                    }
                }
            }

            n := xor_bytes(mut local_dst, local_src, x.out[x.out_used..])

            local_dst = local_dst[n..]
            local_src = local_src[n..]
            x.out_used += n
        }
    }
}