zyna-db/src/index_codec.zig

/// Secondary index entry encoding
/// Index entries store pointers to primary keys, not full items
const std = @import("std");

/// Encode a primary key reference for storage in an index entry
/// Format: [pk_len:varint][pk:bytes][sk_len:varint][sk:bytes]?
/// Returns owned slice that caller must free
pub fn encodePrimaryKeyRef(
    allocator: std.mem.Allocator,
    pk_value: []const u8,
    sk_value: ?[]const u8,
) ![]u8 {
    var buf = std.ArrayList(u8).init(allocator);
    errdefer buf.deinit();
    const writer = buf.writer();

    // Encode partition key
    try encodeVarint(writer, pk_value.len);
    try writer.writeAll(pk_value);

    // Encode sort key if present
    if (sk_value) |sk| {
        try encodeVarint(writer, sk.len);
        try writer.writeAll(sk);
    }

    return buf.toOwnedSlice();
}

/// Decode a primary key reference from an index entry
/// Returns struct with owned slices that caller must free
pub fn decodePrimaryKeyRef(allocator: std.mem.Allocator, data: []const u8) !PrimaryKeyRef {
    var decoder = BinaryDecoder.init(data);

    // Decode partition key
    const pk_len = try decoder.readVarint();
    const pk = try decoder.readBytes(pk_len);
    const owned_pk = try allocator.dupe(u8, pk);
    errdefer allocator.free(owned_pk);

    // Decode sort key if present
    var owned_sk: ?[]u8 = null;
    if (decoder.hasMore()) {
        const sk_len = try decoder.readVarint();
        const sk = try decoder.readBytes(sk_len);
        owned_sk = try allocator.dupe(u8, sk);
    }

    return PrimaryKeyRef{
        .pk = owned_pk,
        .sk = owned_sk,
    };
}

pub const PrimaryKeyRef = struct {
    pk: []u8,
    sk: ?[]u8,

    pub fn deinit(self: *PrimaryKeyRef, allocator: std.mem.Allocator) void {
        allocator.free(self.pk);
        if (self.sk) |sk| allocator.free(sk);
    }
};

// ============================================================================
// Binary Decoder Helper
// ============================================================================

const BinaryDecoder = struct {
    data: []const u8,
    pos: usize,

    pub fn init(data: []const u8) BinaryDecoder {
        return .{ .data = data, .pos = 0 };
    }

    pub fn readBytes(self: *BinaryDecoder, len: usize) ![]const u8 {
        if (self.pos + len > self.data.len) return error.UnexpectedEndOfData;
        const bytes = self.data[self.pos .. self.pos + len];
        self.pos += len;
        return bytes;
    }

    pub fn readVarint(self: *BinaryDecoder) !usize {
        var result: usize = 0;
        var shift: u6 = 0;

        while (self.pos < self.data.len) {
            const byte = self.data[self.pos];
            self.pos += 1;

            result |= @as(usize, byte & 0x7F) << shift;

            if ((byte & 0x80) == 0) {
                return result;
            }

            shift += 7;
            if (shift >= 64) return error.VarintOverflow;
        }

        return error.UnexpectedEndOfData;
    }

    pub fn hasMore(self: *BinaryDecoder) bool {
        return self.pos < self.data.len;
    }
};

// ============================================================================
// Varint encoding (consistent with key_codec and item_codec)
// ============================================================================

fn encodeVarint(writer: anytype, value: usize) !void {
    var v = value;
    while (true) {
        const byte = @as(u8, @intCast(v & 0x7F));
        v >>= 7;

        if (v == 0) {
            try writer.writeByte(byte);
            return;
        } else {
            try writer.writeByte(byte | 0x80);
        }
    }
}