Day 04 - original solution

04.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+ng=0; no=0
+IFS=",-"
+while read -r l; do
+	set -- $l
+	r1=$(($2-$1))
+	r2=$(($4-$3))
+	d=$((r2-r1))
+	o=$(($3-$1))
+	e=$((d+o))
+
+	# p1
+	{ { [ "$e" -ge 0 ] && [ "$e" -le "$d" ]; } || { [ "$e" -le 0 ] && [ "$e" -ge "$d" ]; }; } && ng=$((ng+1))
+
+	# p2
+	{ { [ "$o" -ge 0 ] && [ "$o" -le "$r1" ]; } || { [ "$o" -lt 0 ] && [ "${o#-}" -le "$r2" ]; }; } && no=$((no+1))
+done < 04_input
+echo "$ng"
+echo "$no"

README.md

@@ -8,3 +8,4 @@ this repo will also include online solutions that I think are neat, indicated by
 1. awk (uses features exclusive to GNU's implementation, gawk)
 2. shell (completely POSIX-conforming implementation)
 3. awk (POSIX-conforming)
+4. shell (POSIX, no utilities except `echo` and `read` used)