src/net/http/internal/http3/varint.go

// Copyright 2026 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// The functions in this file are copied from golang.org/x/net/internal/quic/quicwire.
package http3

// consumeVarint parses a variable-length integer, reporting its length.
// It returns a negative length upon an error.
//
// https://www.rfc-editor.org/rfc/rfc9000.html#section-16
func consumeVarint(b []byte) (v uint64, n int) {
	if len(b) < 1 {
		return 0, -1
	}
	b0 := b[0] & 0x3f
	switch b[0] >> 6 {
	case 0:
		return uint64(b0), 1
	case 1:
		if len(b) < 2 {
			return 0, -1
		}
		return uint64(b0)<<8 | uint64(b[1]), 2
	case 2:
		if len(b) < 4 {
			return 0, -1
		}
		return uint64(b0)<<24 | uint64(b[1])<<16 | uint64(b[2])<<8 | uint64(b[3]), 4
	case 3:
		if len(b) < 8 {
			return 0, -1
		}
		return uint64(b0)<<56 | uint64(b[1])<<48 | uint64(b[2])<<40 | uint64(b[3])<<32 | uint64(b[4])<<24 | uint64(b[5])<<16 | uint64(b[6])<<8 | uint64(b[7]), 8
	}
	return 0, -1
}

// consumeVarintInt64 parses a variable-length integer as an int64.
func consumeVarintInt64(b []byte) (v int64, n int) {
	u, n := consumeVarint(b)
	// QUIC varints are 62-bits large, so this conversion can never overflow.
	return int64(u), n
}

// appendVarint appends a variable-length integer to b.
//
// https://www.rfc-editor.org/rfc/rfc9000.html#section-16
func appendVarint(b []byte, v uint64) []byte {
	switch {
	case v <= 63:
		return append(b, byte(v))
	case v <= 16383:
		return append(b, (1<<6)|byte(v>>8), byte(v))
	case v <= 1073741823:
		return append(b, (2<<6)|byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
	case v <= 4611686018427387903:
		return append(b, (3<<6)|byte(v>>56), byte(v>>48), byte(v>>40), byte(v>>32), byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
	default:
		panic("varint too large")
	}
}

// sizeVarint returns the size of the variable-length integer encoding of f.
func sizeVarint(v uint64) int {
	switch {
	case v <= 63:
		return 1
	case v <= 16383:
		return 2
	case v <= 1073741823:
		return 4
	case v <= 4611686018427387903:
		return 8
	default:
		panic("varint too large")
	}
}