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std/encoding/varint.ts

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// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.// Copyright 2020 Keith Cirkel. All rights reserved. MIT license.// Copyright 2023 Skye "MierenManz". All rights reserved. MIT license./** * Functions for encoding typed integers in array buffers. * * @module */
// This implementation is a port of https://deno.land/x/varint@v2.0.0 by @keithamus// This module is browser compatible.
export const MaxUInt64 = 18446744073709551615n;export const MaxVarIntLen64 = 10;export const MaxVarIntLen32 = 5;
const MSB = 0x80;const REST = 0x7f;const SHIFT = 7;const MSBN = 0x80n;const SHIFTN = 7n;
// ArrayBuffer and TypedArray's for "pointer casting"const AB = new ArrayBuffer(8);const U32_VIEW = new Uint32Array(AB);const U64_VIEW = new BigUint64Array(AB);
/** * Given a non empty `buf`, starting at `offset` (default: 0), begin decoding bytes as * VarInt encoded bytes, for a maximum of 10 bytes (offset + 10). The returned * tuple is of the decoded varint 32-bit number, and the new offset with which * to continue decoding other data. * * If a `bigint` in return is undesired, the `decode32` function will return a * `number`, but this should only be used in cases where the varint is * _assured_ to be 32-bits. If in doubt, use `decode()`. * * To know how many bytes the VarInt took to encode, simply negate `offset` * from the returned new `offset`. */export function decode(buf: Uint8Array, offset = 0): [bigint, number] { // Clear the last result from the Two's complement view U64_VIEW[0] = 0n;
// Setup the initiat state of the function let intermediate = 0; let position = 0; let i = offset;
// If the buffer is empty Throw if (buf.length === 0) throw new RangeError("Cannot read empty buffer");
let byte; do { // Get a single byte from the buffer byte = buf[i];
// 1. Take the lower 7 bits of the byte. // 2. Shift the bits into the correct position. // 3. Bitwise OR it with the intermediate value // QUIRK: in the 5th (and 10th) iteration of this loop it will overflow on the shift. // This causes only the lower 4 bits to be shifted into place and removing the upper 3 bits intermediate |= (byte & 0b01111111) << position;
// If position is 28 // it means that this iteration needs to be written the the two's complement view // This only happens once due to the `-4` in this branch if (position === 28) { // Write to the view U32_VIEW[0] = intermediate; // set `intermediate` to the remaining 3 bits // We only want the remaining three bits because the other 4 have been "consumed" on line 21 intermediate = (byte & 0b01110000) >>> 4; // set `position` to -4 because later 7 will be added, making it 3 position = -4; }
// Increment the shift position by 7 position += 7; // Increment the iterator by 1 i++; // Keep going while there is a continuation bit } while ((byte & 0b10000000) === 0b10000000); // subtract the initial offset from `i` to get the bytes read const nRead = i - offset;
// If 10 bytes have been read and intermediate has overflown // it means that the varint is malformed // If 11 bytes have been read it means that the varint is malformed // If `i` is bigger than the buffer it means we overread the buffer and the varint is malformed if ((nRead === 10 && intermediate > -1) || nRead === 11 || i > buf.length) { throw new RangeError("malformed or overflow varint"); }
// Write the intermediate value to the "empty" slot // if the first slot is taken. Take the second slot U32_VIEW[Number(nRead > 4)] = intermediate;
return [U64_VIEW[0], i];}
/** * Given a `buf`, starting at `offset` (default: 0), begin decoding bytes as * VarInt encoded bytes, for a maximum of 5 bytes (offset + 5). The returned * tuple is of the decoded varint 32-bit number, and the new offset with which * to continue decoding other data. * * VarInts are _not 32-bit by default_ so this should only be used in cases * where the varint is _assured_ to be 32-bits. If in doubt, use `decode()`. * * To know how many bytes the VarInt took to encode, simply negate `offset` * from the returned new `offset`. */export function decode32(buf: Uint8Array, offset = 0): [number, number] { for ( let i = offset, len = Math.min(buf.length, offset + MaxVarIntLen32), shift = 0, decoded = 0; i <= len; i += 1, shift += SHIFT ) { const byte = buf[i]; decoded += (byte & REST) * Math.pow(2, shift); if (!(byte & MSB)) return [decoded, i + 1]; } throw new RangeError("malformed or overflow varint");}
/** * Takes unsigned number `num` and converts it into a VarInt encoded * `Uint8Array`, returning a tuple consisting of a `Uint8Array` slice of the * encoded VarInt, and an offset where the VarInt encoded bytes end within the * `Uint8Array`. * * If `buf` is not given then a Uint8Array will be created. * `offset` defaults to `0`. * * If passed `buf` then that will be written into, starting at `offset`. The * resulting returned `Uint8Array` will be a slice of `buf`. The resulting * returned number is effectively `offset + bytesWritten`. */export function encode( num: bigint | number, buf: Uint8Array = new Uint8Array(MaxVarIntLen64), offset = 0,): [Uint8Array, number] { num = BigInt(num); if (num < 0n) throw new RangeError("signed input given"); for ( let i = offset, len = Math.min(buf.length, MaxVarIntLen64); i <= len; i += 1 ) { if (num < MSBN) { buf[i] = Number(num); i += 1; return [buf.slice(offset, i), i]; } buf[i] = Number((num & 0xFFn) | MSBN); num >>= SHIFTN; } throw new RangeError(`${num} overflows uint64`);}