// encoding/base64 — RFC 4648 base64 encode/decode, buffer-based.
//
// Mirrors Hare's encoding::base64 surface, modulo Hare's stream-based
// encoder/decoder. ww ships the in-memory subset only: `encode(dst,
// src)` writes the encoded bytes into `dst`, returning the count;
// `decode(dst, src)` writes the decoded bytes into `dst`, returning a
// count or invalid.
//
// std uses '+' and '/' for indexes 62 and 63 (the RFC 4648 §4
// alphabet); url uses '-' and '_' (the §5 url-safe alphabet). Both
// pad encoded output with '=' to a multiple of 4 bytes.

// invalid — input was not well-formed base64 (bad char, wrong length,
// padding error). Payload is the byte index of the first offending
// position. Matches Hare's errors::invalid pairing with strconv.
export type invalid = !i32;

// encodedsize — bytes required to encode `n` source bytes (including
// '=' padding). Hare names it the same.
export fn encodedsize(n: i32) i32 = {
	if (n == 0) { return 0; };
	return ((n - 1) / 3 + 1) * 4;
};

// decodedsize — upper bound on the number of bytes decoded from `n`
// encoded bytes. The exact count depends on padding; callers consult
// the i32 returned by `decode`.
export fn decodedsize(n: i32) i32 = {
	return (n / 4) * 3;
};

// encchar — map a 6-bit value to its alphabet character. `urlsafe`
// chooses '-'/'_' instead of '+'/'/' for 62/63.
fn encchar(v: u8, urlsafe: bool) u8 = {
	if (v < 26u8) { return v + 65u8; };		// 'A' + v
	if (v < 52u8) { return v + 71u8; };		// 'a' + (v - 26)  = v + 71
	if (v < 62u8) { return v - 4u8; };		// '0' + (v - 52)  = v - 4
	if (v == 62u8) {
		if (urlsafe) { return 45u8; };		// '-'
		return 43u8;				// '+'
	};
	if (urlsafe) { return 95u8; };			// '_'
	return 47u8;					// '/'
};

// decchar — inverse of encchar. Returns 0..63 on success or 255 on
// invalid char. '=' is handled in the decode loop, not here.
fn decchar(c: u8, urlsafe: bool) u8 = {
	if (c >= 65u8) { if (c <= 90u8)  { return c - 65u8; }; };	// 'A'..'Z'
	if (c >= 97u8) { if (c <= 122u8) { return c - 71u8; }; };	// 'a'..'z'
	if (c >= 48u8) { if (c <= 57u8)  { return c + 4u8; }; };	// '0'..'9'
	if (urlsafe) {
		if (c == 45u8) { return 62u8; };	// '-'
		if (c == 95u8) { return 63u8; };	// '_'
	} else {
		if (c == 43u8) { return 62u8; };	// '+'
		if (c == 47u8) { return 63u8; };	// '/'
	};
	return 255u8;
};

// encodeinto — encode `src` into `dst` using the std (`urlsafe=false`)
// or url-safe (`urlsafe=true`) alphabet. `dst` must hold at least
// encodedsize(src.len) bytes. Returns the number of bytes written.
fn encodeinto(dst: []u8, src: []u8, urlsafe: bool) i32 = {
	let i: i32 = 0;
	let j: i32 = 0;
	for (i + 2 < src.len) {
		let b0: u8 = src[i];
		let b1: u8 = src[i + 1];
		let b2: u8 = src[i + 2];
		dst[j]     = encchar(b0 >> 2u8, urlsafe);
		dst[j + 1] = encchar(((b0 & 3u8) << 4u8) | (b1 >> 4u8), urlsafe);
		dst[j + 2] = encchar(((b1 & 15u8) << 2u8) | (b2 >> 6u8), urlsafe);
		dst[j + 3] = encchar(b2 & 63u8, urlsafe);
		i += 3;
		j += 4;
	};
	let rem: i32 = src.len - i;
	if (rem == 1) {
		let b0: u8 = src[i];
		dst[j]     = encchar(b0 >> 2u8, urlsafe);
		dst[j + 1] = encchar((b0 & 3u8) << 4u8, urlsafe);
		dst[j + 2] = 61u8;	// '='
		dst[j + 3] = 61u8;	// '='
		j += 4;
	};
	if (rem == 2) {
		let b0: u8 = src[i];
		let b1: u8 = src[i + 1];
		dst[j]     = encchar(b0 >> 2u8, urlsafe);
		dst[j + 1] = encchar(((b0 & 3u8) << 4u8) | (b1 >> 4u8), urlsafe);
		dst[j + 2] = encchar((b1 & 15u8) << 2u8, urlsafe);
		dst[j + 3] = 61u8;	// '='
		j += 4;
	};
	return j;
};

// encode — encode `src` into `dst` using the std alphabet. Returns
// the number of bytes written. `dst` must hold at least
// encodedsize(src.len) bytes.
export fn encode(dst: []u8, src: []u8) i32 = {
	return encodeinto(dst, src, false);
};

// encodeurl — same as encode but uses the url-safe alphabet ('-'/'_'
// for 62/63).
export fn encodeurl(dst: []u8, src: []u8) i32 = {
	return encodeinto(dst, src, true);
};

// decodeinto — decode base64 `src` into `dst`. `dst` must hold at
// least decodedsize(src.len) bytes. Returns the number of bytes
// written, or invalid with the offending source index.
fn decodeinto(dst: []u8, src: []u8, urlsafe: bool) (i32 | invalid) = {
	if (src.len == 0) { return 0; };
	if ((src.len & 3) != 0) { return src.len: invalid; };
	let i: i32 = 0;
	let j: i32 = 0;
	let end: i32 = src.len;
	for (i < end) {
		let c0: u8 = src[i];
		let c1: u8 = src[i + 1];
		let c2: u8 = src[i + 2];
		let c3: u8 = src[i + 3];
		let v0: u8 = decchar(c0, urlsafe);
		let v1: u8 = decchar(c1, urlsafe);
		if (v0 == 255u8) { return i: invalid; };
		if (v1 == 255u8) { return (i + 1): invalid; };
		// Last quad may carry '=' padding.
		if (i + 4 == end) {
			if (c2 == 61u8) {
				// "XX=="
				if (c3 != 61u8) { return (i + 3): invalid; };
				dst[j] = (v0 << 2u8) | (v1 >> 4u8);
				j += 1;
				i += 4;
				return j;
			};
			let v2: u8 = decchar(c2, urlsafe);
			if (v2 == 255u8) { return (i + 2): invalid; };
			if (c3 == 61u8) {
				// "XXX="
				dst[j]     = (v0 << 2u8) | (v1 >> 4u8);
				dst[j + 1] = (v1 << 4u8) | (v2 >> 2u8);
				j += 2;
				i += 4;
				return j;
			};
			let v3: u8 = decchar(c3, urlsafe);
			if (v3 == 255u8) { return (i + 3): invalid; };
			dst[j]     = (v0 << 2u8) | (v1 >> 4u8);
			dst[j + 1] = (v1 << 4u8) | (v2 >> 2u8);
			dst[j + 2] = (v2 << 6u8) | v3;
			j += 3;
			i += 4;
			return j;
		};
		let v2: u8 = decchar(c2, urlsafe);
		let v3: u8 = decchar(c3, urlsafe);
		if (v2 == 255u8) { return (i + 2): invalid; };
		if (v3 == 255u8) { return (i + 3): invalid; };
		dst[j]     = (v0 << 2u8) | (v1 >> 4u8);
		dst[j + 1] = (v1 << 4u8) | (v2 >> 2u8);
		dst[j + 2] = (v2 << 6u8) | v3;
		i += 4;
		j += 3;
	};
	return j;
};

// decode — decode std-alphabet base64 from `src` into `dst`. Returns
// the count of decoded bytes, or invalid on a malformed input.
export fn decode(dst: []u8, src: []u8) (i32 | invalid) = {
	return decodeinto(dst, src, false);
};

// decodeurl — same as decode but accepts the url-safe alphabet.
export fn decodeurl(dst: []u8, src: []u8) (i32 | invalid) = {
	return decodeinto(dst, src, true);
};