Initial data commit

Task/Modular-inverse/M4/modular-inverse.m4

@@ -0,0 +1,18 @@
+divert(-1)
+
+# I assume non-negative arguments. The algorithm is described at
+# https://en.wikipedia.org/w/index.php?title=Extended_Euclidean_algorithm&oldid=1135569411#Modular_integers
+
+define(`inverse',`_$0(eval(`$1'), eval(`$2'))')
+define(`_inverse',`_$0($2, 0, 1, $2, $1)')
+define(`__inverse',
+`dnl  n = $1,  t = $2,  newt = $3,  r = $4,  newr = $5
+ifelse(eval($5 != 0), 1, `$0($1, $3,
+                             eval($2 - (($4 / $5) * $3)),
+                             $5,eval($4 % $5))',
+       eval($4 > 1), 1, `no inverse',
+       eval($2 < 0), 1, eval($2 + $1),
+       $2)')
+
+divert`'dnl
+inverse(42, 2017)