A-M baby

Task/Modular-inverse/Tcl/modular-inverse-1.tcl

@@ -0,0 +1,17 @@
+proc gcdExt {a b} {
+    if {$b == 0} {
+	return [list 1 0 $a]
+    }
+    set q [expr {$a / $b}]
+    set r [expr {$a % $b}]
+    lassign [gcdExt $b $r] s t g
+    return [list $t [expr {$s - $q*$t}] $g]
+}
+proc modInv {a m} {
+    lassign [gcdExt $a $m] i -> g
+    if {$g != 1} {
+	return -code error "no inverse exists of $a %! $m"
+    }
+    while {$i < 0} {incr i $m}
+    return $i
+}

Task/Modular-inverse/Tcl/modular-inverse-2.tcl

@@ -0,0 +1,4 @@
+puts "42 %! 2017 = [modInv 42 2017]"
+catch {
+    puts "2 %! 4 = [modInv 2 4]"
+} msg; puts $msg