#!/bin/sh

# Edit a file without changing its timestamps.
# Both access and modification times are maintained,
# or optionally those timestamps are set to a specific epoch.

# License: LGPLv2
# Author:
#    http://www.pixelbeat.org/
# Notes:
#    $EDITOR if set, must not fork at startup, so if you
#    want to use gvim for example, ensure EDITOR="gvim -f".
#    Currently linux can read timestamps with nanosecond resolution
#    but only set a specific timestamp with microsecond resolution
#    (if the filesystem even supports that).
# Changes:
#    V1.0, 31 May 2006, Initial release
#    V1.1, 13 Jul 2007, Allow specifying epoch
#                       Remove the use of temp files
#    V1.2, 24 Jul 2007, Fallback to second resolution for touch implementations
#                       that don't handle nanoseconds in the date string.
#                       Warn if nanosecond resolution is lost due to limitations
#                       in `touch`, the libc/kernel interface or the filesystem.
#    V1.3, 05 Feb 2009, Don't allow to specify epoch with leading + or -
#                       as it's common to call vim like: vim file +line_num etc.
#                       Only allow 2 parameters rather than ignoring extra ones.

setglobal file = $1
setglobal epoch = $2

if test ! -f $file || test $Argc -ne 1 -a $Argc -ne 2 {
    echo "Usage: $[basename $0] file [epoch]" >&2
    exit 1
}

if test ! $epoch {
    setglobal epoch = $[date --reference="$file" +%s.%N] || exit 1
} else {
    if echo $epoch | grep -Eq "^[\+-]" {
        echo "Epochs with leading +/- are ambiguous with editor options" >&2
        exit 1
    }
    if ! date --date="1970-01-01 UTC $epoch seconds" >/dev/null 2>&1 {
        echo "Invalid epoch specified [$epoch]" >&2
        exit 1
    }
}

if echo $epoch | grep -Fq "." { #valid nanosecond format
    setglobal seconds = $[echo $epoch | cut -d. -f1]
    if echo $epoch | grep -Eq "\.0+$" { #strip redundant nanoseconds
        setglobal epoch = $seconds #since touch may not support nanosecond format
    } else {
        setglobal checkns = '"true'" #need to check nanosecond portion later
    }
}
$(EDITOR:-vim) $file
test ! $seconds && setglobal err = '"/dev/tty'" || setglobal err = '"/dev/null'"
touch $file --date="1970-01-01 UTC $epoch seconds" 2>$err
if test $Status -ne 0 -a $seconds { #maybe touch doesn't support nanoseconds
    touch $file --date="1970-01-01 UTC $seconds seconds" &&
    echo "Warning: sub second portion of timestamp ignored" >&2
} else {
    if test $checkns {
        setglobal new_ts = $[date --reference="$file" +%s.%N]
        setglobal diff = $[echo "(($epoch-$new_ts)*10^9)/1" | bc]
        if test $diff -ne 0 {
            echo "Warning: timestamp set $(diff)ns backwards" >&2
        }
    }
}

#Hmm could have option to inc time by 1 second
#so that updates would be noticed as normal
#but the relative ordering of a file in time
#in relation to other files would probably be unchanged