#illusory0x0/fmt

#Key Features

#1. Destination Passing Style API

test "basic formatting" {
  let buffer = Array::make(1024, Byte::default())
  fn str(fmt : Bytes, data : Array[&Format]) -> String raise {
    let offset = @fmt.write(buffer, fmt, data)
    ascii_bytes_to_string_helper(buffer[0:offset])
  }

  inspect(try? str(b"hello {} world", [123]), content="Ok(\"hello 123 world\")")
}

fn ascii_bytes_to_string_helper(avb : ArrayView[Byte]) -> String {
  let buf = StringBuilder::new()
  for b in avb {
    buf.write_char(b.to_char())
  }
  buf.to_string()
}

#2. Memory Allocation with count

test "count allocation" {
  let value = 12345
  let buffer_size = Format::count(value)
  let buffer = Array::make(buffer_size, Byte::default())
  let written = Format::write(value, buffer)
  inspect(written <= buffer_size, content="true")
}

#3. Write Method with Return Size

test "write with return size" {
  let buffer = Array::make(10, Byte::default())
  let written = Format::write(42, buffer)
  inspect(written, content="2") // "42" uses 2 bytes
}

#4. Bump Pointer Memory Management

#5. Extensible API Design

#Extensible API Pattern

pub(all) struct BigEndian[T](T)

pub(open) trait BigEndianFormat {
  /// The number of bytes written must not exceed count().
  write(Self, ArrayView[Byte]) -> Int
  /// Upper bound on the number of bytes written
  count(Self) -> Int
}

pub impl[T : BigEndianFormat] Format for BigEndian[T] with write(value, buffer) {
  T::write(value.inner(), buffer)
}

pub impl[T : BigEndianFormat] Format for BigEndian[T] with count(self) {
  T::count(self.inner())
}

#How It Works

• BigEndian[T] is a wrapper type that holds a value of type T
• BigEndianFormat is a trait that defines the specific behavior for big-endian formatting
• The Format trait is implemented for BigEndian[T] only when T implements BigEndianFormat
• This forwards the method calls to the specific BigEndianFormat implementation

1. Customize behavior for specific endianness formats
2. Maintain type safety
3. Provide different implementations for the same base type
4. Keep the API clean and extensible

#Core API

#Format Trait

pub(open) trait Format {
  /// The number of bytes written must not exceed count().
  write(Self, ArrayView[Byte]) -> Int
  /// Upper bound on the number of bytes written
  count(Self) -> Int
}

#Formatting Function

pub fn write(
  buffer : ArrayView[Byte],
  format : Bytes,
  data : Array[&Format],
) -> ArrayView[Byte] raise

#Supported Types

• Basic Types: Int, UInt, Double
• Endianness Wrappers: BigEndian[T], LittleEndian[T]
• Custom Types: Through trait implementation

#Examples

#Basic Formatting

test "string interpolation" {
  let buffer = Array::make(1024, Byte::default())
  fn str(fmt : Bytes, data : Array[&Format]) -> String raise {
    let offset = @fmt.write(buffer, fmt, data)
    ascii_bytes_to_string_helper(buffer[0:offset])
  }

  inspect(
    try? str(b"Value: {}, Count: {}", [42, 10]),
    content="Ok(\"Value: 42, Count: 10\")",
  )
}

#Endianness-Specific Formatting

test "endianness formatting" {
  let buffer = Array::make(8, Byte::default())
  let big_endian_int = @fmt.BigEndian(0x80818283)
  let written = Format::write(big_endian_int, buffer)
  inspect(written, content="4")
  // Buffer now contains [0x80, 0x81, 0x82, 0x83]
}

#Custom Type Implementation

struct Point {
  x : Int
  y : Int
}

impl Format for Point with write(self, buf) {
  let mut offset = 0
  buf[offset] = '('
  offset += 1

  offset += Format::write(self.y, buf[offset:])

  buf[offset] = ','
  offset += 1

  offset += Format::write(self.x, buf[offset:])

  buf[offset] = ')'
  offset += 1
  offset
}

impl Format for Point with count(self) {
  3 + Format::count(self.x) + Format::count(self.y)
}

test {
  let point = Point:: { x: 1, y: 2 };
  let buffer = Array::make(1024, Byte::default());
  fn str(fmt : Bytes, data : Array[&Format]) -> String raise {
    let offset = @fmt.write(buffer, fmt, data)
    ascii_bytes_to_string_helper(buffer[0:offset])
  }
  inspect(try? str("{}",[point]), content=(#|Ok("(2,1)")
  ))
}

#Error Handling

suberror DebugError String derive(Show)

• Buffer overflow when the provided buffer is too small
• Placeholder count mismatch in format strings

#Performance Benefits

1. Zero-copy: Direct writing to destination buffer
2. Predictable allocation: Upper bounds provided by count()
3. Minimal overhead: Bump pointer allocation strategy
4. Type-safe: Compile-time checking of format implementations