Add Euclidean GCD in Factor (#425)

* Factor Euclidean GCD

* added .editorconfig changes

* update file to follow new editorconfig

* vim, stop adding tabs, kthxbye

* added absolute values

Author: Nic Hartley

.editorconfig

@@ -133,6 +133,11 @@ indent_size = 2
 indent_style = space
 indent_size = 4
 
+# Factor
+[*.factor]
+intent_style = space
+indent_size = 2
+
 #Lua
 [*.lua]
 indent_style = space

book.json

@@ -144,6 +144,10 @@
           "lang": "f90",
           "name": "Fortran90"
         },
+        {
+          "lang": "factor",
+          "name": "Factor"
+        },
         {
           "lang": "ws",
           "name": "Whitespace"

contents/euclidean_algorithm/code/factor/euclid.factor

@@ -0,0 +1,32 @@
+: euclid- ( a b -- gcd )
+  [ abs ] bi@
+  [ 2dup = ]
+  [
+    ! make sure the lower number is deeper
+    2dup >= [ swap ] when
+    over -
+    ! leaves us with stack { <lower> <greater - lower> }
+  ]
+  until
+  ! we have the GCD twice now, drop one
+  drop
+;
+
+: euclid% ( a b -- gcd )
+  [ abs ] bi@   ! take both absolute values
+  [ dup zero? ] ! check if `b` (on top) is 0
+  [
+    ! a b -> a b b -> b a b -> b a%b
+    dup -rot mod
+  ]
+  until
+  ! the zero is on top, so get rid of it
+  drop
+;
+
+42 56 euclid% .  ! 14
+48 180 euclid% . ! 12
+
+42 56 euclid- .  ! 14
+48 180 euclid- . ! 12
+

contents/euclidean_algorithm/euclidean_algorithm.md

@@ -41,6 +41,8 @@ The algorithm is a simple way to find the *greatest common divisor* (GCD) of two
 [import:13-24, lang="nim"](code/nim/euclid_algorithm.nim)
 {% sample lang="f90" %}
 [import:1-19, lang="fortran"](code/fortran/euclidean.f90)
+{% sample lang="factor" %}
+[import:1-13, lang="factor"](code/factor/euclid.factor)
 {% sample lang="ws" %}
 [import, lang="whitespace"](code/whitespace/euclidian_sub.ws)
 {% sample lang="scala" %}
@@ -100,6 +102,8 @@ Modern implementations, though, often use the modulus operator (%) like so
 [import:1-11, lang="nim"](code/nim/euclid_algorithm.nim)
 {% sample lang="f90" %}
 [import:21-34, lang="fortran"](code/fortran/euclidean.f90)
+{% sample lang="factor" %}
+[import:15-25, lang="factor"](code/factor/euclid.factor)
 {% sample lang="ws" %}
 [import, lang="whitespace"](code/whitespace/euclidian_mod.ws)
 {% sample lang="scala" %}
@@ -164,6 +168,8 @@ The Euclidean Algorithm is truly fundamental to many other algorithms throughout
 [import, lang="nim" %](code/nim/euclid_algorithm.nim)
 {% sample lang="f90" %}
 [import, lang="fortran"](code/fortran/euclidean.f90)
+{% sample lang="factor" %}
+[import, lang="factor"](code/factor/euclid.factor)
 {% sample lang="ws" %}
 Here is a readable version of the algorithms with comments. First, subtraction method:
 [import, lang="whitespace"](code/whitespace/euclidian_sub_comments.ws)