module runtime

import os

// used_memory retrieves the current physical memory usage of the process.
@[manualfree]
pub fn used_memory() !u64 {
    buffer := [1024]u8{}
    pc := unsafe { &buffer[0] }
    // see https://man7.org/linux/man-pages/man5/proc_pid_stat.5.html for a detailed description of the format.
    // Here is an example:
    // 699989 (test_program) R 178038 699989 178038 34816 699989 0 102 0 0 0 0 0 0 0 20 0 1 0 84188763 5726208 129 18446744073709551615 93824992239616 93824992257073 140737488345056 0 0 0 0 0 0 0 0 0 17 0 0 0 0 0 0 93824992275088 93824992276672 93824992280576 140737488346201 140737488346221 140737488346221 140737488351211 0
    // => read the file in the fixed buffer, search to after the `)` to skip the variable name part, then scan till the `129` RSS value, and then convert it from pages to bytes.
    mut f := os.open_file('/proc/self/stat', 'r')!
    defer { f.close() }
    read := f.read_into_ptr(pc, buffer.len)!
    if read <= 0 {
        return error('could not read from /proc/self/stat')
    }
    mut c := 0
    unsafe {
        // scan to after the process name (`comm`) field:
        for c < buffer.len && *pc != `)` {
            pc++
            c++
        }
        mut spaces := 0
        // the following fields, are space separated numbers, skip past them to the `rss` field:
        for c < buffer.len && spaces < 22 {
            if *pc == ` ` {
                spaces++
            }
            pc++
            c++
        }
        mut ndigits := 0
        // scan till the end of the `rss` field:
        for c < buffer.len && *(pc + ndigits) != ` ` {
            ndigits++
            c++
        }
        rss_pages := pc.vstring_with_len(ndigits).u64()
        page_size := C.sysconf(C._SC_PAGESIZE)
        return u64(rss_pages * page_size)
    }
}