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## rustnim prelude — emitted at the top of every generated module.
##
## Everything here exists to make Nim's observable behaviour match Rust's
## exactly. Where the two languages already agree (signed `shr` is arithmetic
## in both; fixed-width unsigned arithmetic wraps in both; integer `div`/`mod`
## truncate toward zero in both) there is deliberately nothing here: the
## operator is mapped directly and no helper is involved.

# Rust's `&[T]` is a borrowed view, not a copy. Nim's view types model exactly
# that, including returning one from a proc: writing through the returned view
# is visible in the original buffer. Probed against Nim 2.2.4 before relying on
# it, because copying instead would silently change aliasing.
{.experimental: "views".}

import std/[unicode, strutils]

type
  RustPanic* = object of CatchableError

  ## `core::fmt`'s own error and sink, kept distinct from any user type that
  ## happens to be called `Error`. A formatting impl is lowered to a proc that
  ## returns the formatted string, so these appear only in signatures.
  FmtError* = object
  FmtResult* = object
    ok*: bool

  Option*[T] = object
    case has*: bool
    of true: val*: T
    of false: discard

  Result*[T, E] = object
    case ok*: bool
    of true: val*: T
    of false: err*: E

proc rsPanic*(msg: string) {.noreturn.} =
  raise newException(RustPanic, msg)

proc rsSome*[T](v: T): Option[T] = Option[T](has: true, val: v)
proc rsNone*[T](): Option[T] = Option[T](has: false)
proc rsOk*[T, E](v: T): Result[T, E] = Result[T, E](ok: true, val: v)
proc rsErr*[T, E](e: E): Result[T, E] = Result[T, E](ok: false, err: e)

proc rsOkOr*[T, E](o: Option[T], e: E): Result[T, E] =
  if o.has: Result[T, E](ok: true, val: o.val) else: Result[T, E](ok: false, err: e)

proc unwrapOr*[T](o: Option[T], d: T): T =
  if o.has: o.val else: d
proc unwrapOr*[T, E](r: Result[T, E], d: T): T =
  if r.ok: r.val else: d

proc unwrap*[T](o: Option[T]): T =
  if not o.has: rsPanic("called `Option::unwrap()` on a `None` value")
  o.val
proc unwrap*[T, E](r: Result[T, E]): T =
  if not r.ok: rsPanic("called `Result::unwrap()` on an `Err` value")
  r.val

# ---------------------------------------------------------------------------
# Display / Debug.
#
# Rust's `{}` and `{:?}` are two distinct formats and they differ for floats,
# strings, chars and sequences. Nim's `$` matches neither consistently, so both
# are implemented here rather than approximated with `$`.
# ---------------------------------------------------------------------------

proc rsDisplay*(x: SomeInteger): string = $x
proc rsDebug*(x: SomeInteger): string = $x
proc rsDisplay*(x: bool): string = $x
proc rsDebug*(x: bool): string = $x

proc rsFloatStr(x: float64, debug: bool): string =
  ## Nim and Rust both print the shortest round-tripping decimal, but they
  ## spell the result differently in three places.
  if x != x: return "NaN"
  if x == Inf: return "inf"
  if x == -Inf: return "-inf"
  result = $x
  result = result.replace("e+", "e")        # Nim `1e+21`, Rust `1e21`
  if not debug and result.endsWith(".0"):   # Rust Display drops a bare `.0`
    result.setLen(result.len - 2)

proc rsDisplay*(x: float32 | float64): string = rsFloatStr(float64(x), false)
proc rsDebug*(x: float32 | float64): string = rsFloatStr(float64(x), true)

proc rsDisplay*(x: string): string = x
proc rsDebug*(x: string): string =
  result = "\""
  for c in x:
    case c
    of '"': result.add("\\\"")
    of '\\': result.add("\\\\")
    of '\n': result.add("\\n")
    of '\t': result.add("\\t")
    of '\r': result.add("\\r")
    else: result.add(c)
  result.add("\"")

proc rsDisplay*(x: Rune): string = $x
proc rsDebug*(x: Rune): string =
  case $x
  of "'": "'\\''"
  of "\\": "'\\\\'"
  of "\n": "'\\n'"
  of "\t": "'\\t'"
  of "\r": "'\\r'"
  else: "'" & $x & "'"

proc rsDebug*[T](x: seq[T] | openArray[T]): string =
  result = "["
  for i in 0 ..< x.len:
    if i > 0: result.add(", ")
    result.add(rsDebug(x[i]))
  result.add("]")

proc rsDebug*[T](o: Option[T]): string =
  if o.has: "Some(" & rsDebug(o.val) & ")" else: "None"
proc rsDebug*[T, E](r: Result[T, E]): string =
  if r.ok: "Ok(" & rsDebug(r.val) & ")" else: "Err(" & rsDebug(r.err) & ")"

# ---------------------------------------------------------------------------
# Radix formats: `{:x}`, `{:X}`, `{:b}`, `{:o}`, with Rust's width/zero-fill.
# Rust formats the *two's-complement bit pattern*, so a negative i8 prints as
# `ff`, not `-1`. `toHex` on the unsigned view of the same width reproduces it.
# ---------------------------------------------------------------------------

proc rsRadix*[T: SomeInteger](x: T, base: int, upper: bool): string =
  var v: uint64 =
    when T is SomeSignedInt:
      # Sign-extend then mask to the type's own width, so the printed bit
      # pattern is the Rust one for this exact integer type.
      cast[uint64](int64(x)) and (if sizeof(T) == 8: high(uint64) else: (1'u64 shl (sizeof(T) * 8)) - 1)
    else:
      uint64(x)
  if v == 0: return "0"
  const digits = "0123456789abcdef"
  while v > 0'u64:
    result.add(digits[int(v mod uint64(base))])
    v = v div uint64(base)
  for i in 0 ..< result.len div 2:
    swap(result[i], result[result.len - 1 - i])
  if upper: result = result.toUpperAscii()

proc rsPad*(s: string, width: int, zero: bool): string =
  if s.len >= width: return s
  let fill = width - s.len
  if zero and s.len > 0 and s[0] == '-':
    "-" & repeat('0', fill) & s[1 .. ^1]
  elif zero:
    repeat('0', fill) & s
  else:
    repeat(' ', fill) & s

proc rsStrView*(b: openArray[uint8]): openArray[char] =
  ## Rust's `str::from_utf8_unchecked` reinterprets a byte slice as a string
  ## slice without copying or validating. Nim's `char` and `uint8` are both one
  ## byte, so the same view is handed back over the same memory -- writes
  ## through the original are visible here, as they are in Rust.
  if b.len == 0:
    result = toOpenArray(cast[ptr UncheckedArray[char]](nil), 0, -1)
  else:
    result = toOpenArray(cast[ptr UncheckedArray[char]](unsafeAddr b[0]), 0, b.len - 1)

proc rsStringOf*(b: openArray[uint8]): string =
  ## `String::from_utf8_unchecked` takes ownership of the bytes. Nim's `string`
  ## is an owned value, so this copies -- which is what the Rust call does to
  ## the `Vec` it consumes, from the caller's point of view.
  result = newStringOfCap(b.len)
  for v in b: result.add(char(v))

proc rsDisplay*(x: openArray[char]): string =
  result = newStringOfCap(x.len)
  for c in x: result.add(c)

proc rsDebug*(x: openArray[char]): string = rsDebug(rsDisplay(x))

proc rsBytes*(s: string): seq[uint8] =
  ## Rust's `str::as_bytes` is a view of the UTF-8 encoding; Nim's `string` is
  ## already those bytes, so this is a reinterpretation, not a conversion.
  result = newSeq[uint8](s.len)
  for i in 0 ..< s.len: result[i] = uint8(s[i])

proc newSeqWith*[T](n: int, v: T): seq[T] =
  result = newSeq[T](n)
  for i in 0 ..< n: result[i] = v