Decode the protobuf wire format in Gleam!
Decode len field
Gavin Morrow
(Sep 18, 2025, 3:21 PM EDT)
ba88a40a
fdac67e8
+38
-2
+22
-1
src/protobuf_decode_gleam.gleam
+22
-1
src/protobuf_decode_gleam.gleam
···
36
36
InvalidVarInt(leftover_bits: BitArray, acc: BitArray)
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InvalidI64(bits: BitArray)
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UnableToDecode(List(decode.DecodeError))
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+
InvalidLen(len: Int, value_bits: BitArray)
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}
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fn read_fields(
···
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let read_fn: fn(BitArray) -> ValueResult = case wire_type {
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VarInt -> read_varint
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I64 -> read_i64
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-
Len -> todo
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+
Len -> read_len
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I32 -> todo
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}
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use #(value, rest): #(Dynamic, BitArray) <- result.try({
···
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<<i64:bits-size(64), rest:bytes>> -> Ok(#(i64, rest))
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bits -> Error(InvalidI64(bits:))
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}
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+
}
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+
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+
fn read_len(bits: BitArray) -> ValueResult {
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// First, read the length of the value
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+
// It is encoded as a varint immediately after the tag
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use #(len, bits) <- result.try(read_varint(bits))
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+
let len = bit_array_to_uint(len)
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+
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// Just decoded a uint, so should be safe
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assert len > 0
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+
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+
use value <- result.try(
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+
bit_array.slice(from: bits, at: 0, take: len)
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|> result.map_error(fn(_) { InvalidLen(len:, value_bits: bits) }),
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)
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// Assert b/c if the len was too long, it would've errored in the prev slice
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let assert Ok(rest) =
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bit_array.slice(from: bits, at: len, take: bit_array.byte_size(bits) - len)
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+
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Ok(#(value, rest))
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}
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pub fn decode_uint() -> Decoder(Int) {
+16
-1
test/protobuf_decode_gleam_test.gleam
+16
-1
test/protobuf_decode_gleam_test.gleam
···
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}
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fn person_decoder() -> Decoder(Person) {
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+
let person_inner_decoder = {
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+
use bits <- decode.then(decode.bit_array)
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+
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let person = decode(from: bits, using: person_decoder())
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+
case person {
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Ok(person) -> decode.success(person)
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+
Error(_) ->
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decode.failure(
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+
Person(id: 0, age: 0, score: 0, self: option.None),
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+
"Person",
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)
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}
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}
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+
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use id <- decode.field(3, decode_u64())
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use age <- decode.field(1, decode_uint())
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use score <- decode.field(2, decode_uint())
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use person <- decode.optional_field(
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4,
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option.None,
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-
person_decoder() |> decode.map(option.Some),
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+
decode.optional(person_inner_decoder),
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)
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+
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decode.success(Person(id:, age:, score:, self: person))
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}
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