Add support for `indent` (C autoformatter)

[lloeki]

Name: indent
URL: https://www.gnu.org/software/indent/

indent(1) is a program for automatically formatting C code. The GNU variant supports common style shortcuts such as -kr.

This small UNIX trickery makes it a linter as well:

$ cat > main.c <<'EOF'
#include <stdio.h>

void printNumbers(int start, int end)

{
    for (int i = start; i <= end; i++) {
        if (i % 2 == 0) {
            printf("%d is even.\n", i);
        } else {
            printf("%d is odd.\n", i);
        }
    }
}

int main() {
    int start = 1;
    int end = 10;
       printNumbers(start, end);
    return 0;
}
EOF
$ diff -u main.c <(indent -kr -ts1 main.c -o /dev/stdout)
--- main.c	2024-08-08 12:25:23.743927683 +0200
+++ /dev/fd/63	2024-08-08 12:25:26.142459754 +0200
@@ -1,7 +1,6 @@
 #include <stdio.h>
 
 void printNumbers(int start, int end)
-
 {
     for (int i = start; i <= end; i++) {
         if (i % 2 == 0) {
@@ -12,9 +11,10 @@
     }
 }
 
-int main() {
+int main()
+{
     int start = 1;
     int end = 10;
-       printNumbers(start, end);
+    printNumbers(start, end);
     return 0;
 }
$ echo $?
1
$ indent -kr -ts1 main.c
$ diff -u main.c <(indent -kr -ts1 main.c -o /dev/stdout)
$ echo $?
0

There's no message but the diff can be processed to output a simple generic "lint error" on relevant lines which would already go a long way. It could also be processed to suggest fixes.

It can also read from stdin if so desired:

cat main.c | indent -kr -ts1 /dev/stdin -o /dev/stdout

Manuals:

• GNU: https://www.gnu.org/software/indent/manual/
• BSD: https://www.unix.com/man-page/bsd/1/indent/

Both modify in place and produce backup files if no output file is provided, this is probably undesirable so the /dev/stdout trickery might be preferred.

BSD indent does not use -o for output but a mere additional argument instead: GNU vs BSD should be detected to pass things around properly.

BSD indent supports much less features too, notably it does not support common styles like -kr and not all of what's behind -kr so if BSD indent is detected it could be convenient to have high level options to mimic GNU style flags when BSD indent is detected, e.g K&R would be:

indent -nbad -bap -nbc -br -c33 -cd33 -ncdb -ce -ci4 -cli0 -d0 -di1 -nfc1 -i4 -ip -lp -npcs -npsl -nsc -nsob main.c /dev/stdout`

So having a g:ale_c_indent_style = 'kr' would automatically (e.g by setting g:ale_c_indent_options) use -kr with GNU indent and the above with BSD indent.