Competitive programming in Nim

Updated in 2022-10.

In the afternoon, I came cross the Nim programming language again on Lobsters. I first learned some basics of the language in 2015, but had not touched it since then.

"Nim is a statically typed compiled systems programming language. It combines successful concepts from mature languages like Python, Ada and Modula.", according to its website.

Basic features: parametric polymorphism. Advanced features: macros (including term-rewriting macros), compile-time function execution, effect system, concepts

An idea popped into my mind: why not solve some coding challenges in Nim?

As a niche language, it is not supported on many coding challenge websites. Fortunately, the Nim compiler generates C code. With a small amount of work, we can build a self-contained C source file suitable for submission.

Let's take a LeetCode challenge as an example. We write the main algorithm in Nim and use the emit pragma to write a C wrapper.

# Maximum Number of Events That Can Be Attended II
import algorithm

func solution(events: ptr UncheckedArray[ptr UncheckedArray[cint]], eventSize: int, k: int): int {.exportc.} =
  type E = tuple[start,stop,value: int]
  var
    a = newSeq[E](eventSize)
    dp0: seq[int] = newSeq[int](eventSize+1)
    dp1: seq[int] = newSeq[int](eventSize+1)
  for i in 0..<eventSize:
    a[i] = (cast[int](events[i][0]), cast[int](events[i][1]), cast[int](events[i][2]))
  a.sort(proc (x, y: E): int = x.stop - y.stop)
  for _ in 0..<k:
    for i in 0..<eventSize:
      let j = a.lowerBound(a[i].start, proc (x: E, y: int): int = x.stop - y)
      dp1[i+1] = max(dp1[i], dp0[j]+a[i].value)
    swap(dp0, dp1)
  result = dp0[eventSize]

{.emit: """
int maxValue(int** events, int eventsSize, int* eventsColSize, int k) {
  return solution(events, eventsSize, k);
}
""".}

Then, create a self-contained C file with the approach described on https://zeta.su/posts/amalgamating-nim-programs/.

Install opam

Follow https://opam.ocaml.org/doc/Install.html and run opam init.

Build CIL

Clone https://github.com/goblint/cil. Use a modestly recent version of ocaml or pick a recent ocaml release from opam switch list-available.

% opam switch create .
...
% eval $(opam env)
% which dune
/home/ray/Dev/cil/_opam/bin/dune
% dune build

Patch nimbase.h

Copy lib/nimbase.h to ~/Util/Nim. Comment out a visibility attribute.

@@ -196,7 +196,7 @@
 #  define N_LIB_EXPORT_VAR  __declspec(dllexport)
 #  define N_LIB_IMPORT  extern __declspec(dllimport)
 #else
-#  define N_LIB_PRIVATE __attribute__((visibility("hidden")))
+#  define N_LIB_PRIVATE //__attribute__((visibility("hidden")))
 #  if defined(__GNUC__)
 #    define N_CDECL(rettype, name) rettype name
 #    define N_STDCALL(rettype, name) rettype name

In 2021 goblint-CLI did not handle #define _GNU_SOURCE 1 but it can now.

Generate C amalgamation

On Linux the Nim compiler calls gcc by default. We replace gcc with a CIL wrapper to generate a merged C file.

% cat Makefile
.MAKE.MODE := meta curdirOk=1
for-submit.c: a.nim dedup.nim
	nim c -d:danger --mm:arc -d:useMalloc --nimcache:. a.nim
	nim c --run --skipProjCfg dedup.nim > $@T && mv $@T $@
% nim.cfg
gcc.exe="/home/ray/Util/Nim/cilly.sh"
gcc.linkerexe="/home/ray/Util/Nim/cilly.sh"
% cat ~/Util/Nim/cilly.sh
#!/usr/bin/env sh
~/Dev/cil/_build/default/bin/cilly --noPrintLn --merge --keepmerged -I ~/Util/Nim $@
% chmod +x ~/Util/Nim/cilly.sh

Then run nim c -d:danger --gc:arc -d:useMalloc a.nim to generate a_comb.c. -d:useMalloc avoids Nim's own memory manager and can greatly decrease the C code size. There are several choices for --mm, but --mm:arc can genreate the smallest C code.

a_comb.c looks like:

/* Generated by CIL v. 1.8.1 */
/* print_CIL_Input is true */

typedef long ptrdiff_t;
typedef unsigned long size_t;
...
struct _IO_FILE {
   ...
};
...
__inline extern  __attribute__((__nothrow__)) int ( __attribute__((__nonnull__(1),
__leaf__, __gnu_inline__)) atoi)(char const   *__nptr )  __attribute__((__pure__)) ;
__inline extern int ( __attribute__((__nonnull__(1), __leaf__, __gnu_inline__)) atoi)(char const   *__nptr )
{
  long tmp ;

  {
  tmp = strtol((char const   * __restrict  )__nptr, (char ** __restrict  )((char **)((void *)0)),
               10);
  return ((int )tmp);
}
}

The LeetCode environment includes some glibc headers like <stdio.h> which result in some conflicts due to duplicate definitions of struct _IO_FILE and some GNU extern inline functions. Let's write a Nim program to remove the duplicate definitions.

import strutils
var
  body: seq[string] = @[]
  i = 0
for line in lines "a_comb.c":
  body.add(line)

while i < body.len:
  if body[i].startsWith("struct _IO_FILE {"):
    while not body[i].startsWith "}":
      i += 1
    i += 1
    continue
  if body[i].startsWith("__inline extern") or body[i].startsWith("int main("):
    var c = 0
    var changed = false
    i += 1
    while true:
      c += body[i].count('{') - body[i].count('}')
      changed = changed or c > 0
      i += 1
      if c == 0 and changed: break
    continue

  echo body[i]
  i += 1

nim c --run --skipProjCfg --verbosity:0 x > amalgamation.c

Finally, run { echo '/*'; cat a.nim; echo '*/'; cat amalgamation.c; } | xclip -i -selection clipboard and paste the content into the LeetCode editor:) Well, the Nim source is not really needed but it is useful for archive purposes.

Minimization

Now let's decrease the size to make the amalgamation fit into more platforms.

clang-format safely removes whitespace and makes the program smaller.

% cat .clang-format
ColumnLimit:     9999
IndentWidth:     0
ContinuationIndentWidth: 0
SpaceBeforeAssignmentOperators: false
SpaceBeforeParens: Never

To further decrease the source code length, we can shorten function, variables, and type names. See C minifier with Clang.

Old notes

If CIL fails to handle some syntax, use another deduplicator:

#!/usr/bin/env python
from collections import Counter
import re
import sys
l = list(i.rstrip() for i in sys.stdin.readlines())
seen = Counter()
include = set()
i = 0
output = []
intbits = -1

while i < len(l):
    if l[i] == '#define NIM_INTBITS 64' and intbits == -1:
        intbits = i

    if l[i] == '#include "nimbase.h"':
        i += 1
        continue

    m = re.match(r'#include <(.*)>', l[i])
    if m:
        include.add(m.group(1))
        i += 1
        continue

    if l[i].startswith('N_LIB_PRIVATE '):
        l[i] = l[i][14:]

    m = re.match(r'struct (\w+) \{$', l[i])
    if m: seen[m.group(1)] += 1
    if m and seen[m.group(1)] > 1:
        c = 0
        changed = False
        while True:
            c += l[i].count('{') - l[i].count('}')
            changed = changed or c > 0
            i += 1
            if c == 0 and changed: break
        continue

    m = re.match(r'static N_INLINE\(.*, (\w+)\)\(.*\) \{$', l[i])
    if m: seen[m.group(1)] += 1
    if m and seen[m.group(1)] > 1 or l[i].startswith('int main('):
        c = 0
        changed = False
        while True:
            c += l[i].count('{') - l[i].count('}')
            changed = changed or c > 0
            i += 1
            if c == 0 and changed: break
        continue

    output.append(l[i])
    i += 1

with open('/home/ray/Util/Nim/nimbase.min.h') as f:
    nimbase = list(i.rstrip() for i in f.readlines())

output = output[:intbits] + nimbase + output[intbits:]
for i in include:
    print(f'#include <{i}>')
for line in output:
    print(line)

cat ~/.cache/nim/a_r/*.c | ./dedup.py > for-submit.c