From 3f8299430258179f90a15c5cdfefcc6037f6908a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Andr=C3=A1s=20B=20Nagy?=
 <20251272+BNAndras@users.noreply.github.com>
Date: Wed, 10 Apr 2024 05:52:31 -0700
Subject: [PATCH] Add collatz-conjecture (#204)

---
 config.json                                   |  8 ++++
 .../collatz-conjecture/.docs/instructions.md  | 29 ++++++++++++++
 .../practice/collatz-conjecture/.gitignore    | 11 ++++++
 .../collatz-conjecture/.meta/config.json      | 19 ++++++++++
 .../collatz-conjecture/.meta/example.lfe      | 19 ++++++++++
 .../collatz-conjecture/.meta/tests.toml       | 38 +++++++++++++++++++
 .../practice/collatz-conjecture/Makefile      | 21 ++++++++++
 .../practice/collatz-conjecture/rebar.config  | 11 ++++++
 .../practice/collatz-conjecture/rebar.lock    |  8 ++++
 .../src/collatz-conjecture.app.src            | 11 ++++++
 .../src/collatz-conjecture.lfe                |  4 ++
 .../test/collatz-conjecture-tests.lfe         | 23 +++++++++++
 12 files changed, 202 insertions(+)
 create mode 100644 exercises/practice/collatz-conjecture/.docs/instructions.md
 create mode 100644 exercises/practice/collatz-conjecture/.gitignore
 create mode 100644 exercises/practice/collatz-conjecture/.meta/config.json
 create mode 100644 exercises/practice/collatz-conjecture/.meta/example.lfe
 create mode 100644 exercises/practice/collatz-conjecture/.meta/tests.toml
 create mode 100644 exercises/practice/collatz-conjecture/Makefile
 create mode 100644 exercises/practice/collatz-conjecture/rebar.config
 create mode 100644 exercises/practice/collatz-conjecture/rebar.lock
 create mode 100644 exercises/practice/collatz-conjecture/src/collatz-conjecture.app.src
 create mode 100644 exercises/practice/collatz-conjecture/src/collatz-conjecture.lfe
 create mode 100644 exercises/practice/collatz-conjecture/test/collatz-conjecture-tests.lfe

diff --git a/config.json b/config.json
index 33692bde..64551168 100644
--- a/config.json
+++ b/config.json
@@ -137,6 +137,14 @@
         "prerequisites": [],
         "difficulty": 1
       },
+      {
+        "slug": "collatz-conjecture",
+        "name": "Collatz Conjecture",
+        "uuid": "3796230e-f37e-464c-bb22-8dc3d1664aec",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 2
+      },
       {
         "slug": "difference-of-squares",
         "name": "Difference of Squares",
diff --git a/exercises/practice/collatz-conjecture/.docs/instructions.md b/exercises/practice/collatz-conjecture/.docs/instructions.md
new file mode 100644
index 00000000..ba060483
--- /dev/null
+++ b/exercises/practice/collatz-conjecture/.docs/instructions.md
@@ -0,0 +1,29 @@
+# Instructions
+
+The Collatz Conjecture or 3x+1 problem can be summarized as follows:
+
+Take any positive integer n.
+If n is even, divide n by 2 to get n / 2.
+If n is odd, multiply n by 3 and add 1 to get 3n + 1.
+Repeat the process indefinitely.
+The conjecture states that no matter which number you start with, you will always reach 1 eventually.
+
+Given a number n, return the number of steps required to reach 1.
+
+## Examples
+
+Starting with n = 12, the steps would be as follows:
+
+0. 12
+1. 6
+2. 3
+3. 10
+4. 5
+5. 16
+6. 8
+7. 4
+8. 2
+9. 1
+
+Resulting in 9 steps.
+So for input n = 12, the return value would be 9.
diff --git a/exercises/practice/collatz-conjecture/.gitignore b/exercises/practice/collatz-conjecture/.gitignore
new file mode 100644
index 00000000..6dd20ff0
--- /dev/null
+++ b/exercises/practice/collatz-conjecture/.gitignore
@@ -0,0 +1,11 @@
+## -*- conf -*-
+.rebar3
+_build/
+ebin/
+erl_crash.dump
+rebar3.crashdump
+
+tmp
+bin/configlet
+bin/configlet.exe
+CHECKLIST
diff --git a/exercises/practice/collatz-conjecture/.meta/config.json b/exercises/practice/collatz-conjecture/.meta/config.json
new file mode 100644
index 00000000..bbc95e8b
--- /dev/null
+++ b/exercises/practice/collatz-conjecture/.meta/config.json
@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "BNAndras"
+  ],
+  "files": {
+    "solution": [
+      "src/collatz-conjecture.lfe"
+    ],
+    "test": [
+      "test/collatz-conjecture-tests.lfe"
+    ],
+    "example": [
+      ".meta/example.lfe"
+    ]
+  },
+  "blurb": "Calculate the number of steps to reach 1 using the Collatz conjecture.",
+  "source": "An unsolved problem in mathematics named after mathematician Lothar Collatz",
+  "source_url": "https://en.wikipedia.org/wiki/3x_%2B_1_problem"
+}
diff --git a/exercises/practice/collatz-conjecture/.meta/example.lfe b/exercises/practice/collatz-conjecture/.meta/example.lfe
new file mode 100644
index 00000000..59e039d1
--- /dev/null
+++ b/exercises/practice/collatz-conjecture/.meta/example.lfe
@@ -0,0 +1,19 @@
+(defmodule collatz-conjecture
+  (export (steps 1)))
+
+(defun add1 (n) (+ n 1))
+
+(defun zerop (n) (=:= (rem n 2)))
+
+(defun steps (start)
+  (do-steps start 0))
+
+(defun do-steps
+  ((n _) (when (< n 1))
+    (error "Only positive integers are allowed"))
+  ((1 acc)
+    acc)
+  ((n acc) (when (=:= (rem n 2) 0))
+    (do-steps (div n 2) (add1 acc)))
+  ((n acc)
+    (do-steps (add1 (* 3 n)) (add1 acc))))
diff --git a/exercises/practice/collatz-conjecture/.meta/tests.toml b/exercises/practice/collatz-conjecture/.meta/tests.toml
new file mode 100644
index 00000000..cc34e168
--- /dev/null
+++ b/exercises/practice/collatz-conjecture/.meta/tests.toml
@@ -0,0 +1,38 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[540a3d51-e7a6-47a5-92a3-4ad1838f0bfd]
+description = "zero steps for one"
+
+[3d76a0a6-ea84-444a-821a-f7857c2c1859]
+description = "divide if even"
+
+[754dea81-123c-429e-b8bc-db20b05a87b9]
+description = "even and odd steps"
+
+[ecfd0210-6f85-44f6-8280-f65534892ff6]
+description = "large number of even and odd steps"
+
+[7d4750e6-def9-4b86-aec7-9f7eb44f95a3]
+description = "zero is an error"
+include = false
+
+[2187673d-77d6-4543-975e-66df6c50e2da]
+description = "zero is an error"
+reimplements = "7d4750e6-def9-4b86-aec7-9f7eb44f95a3"
+
+[c6c795bf-a288-45e9-86a1-841359ad426d]
+description = "negative value is an error"
+include = false
+
+[ec11f479-56bc-47fd-a434-bcd7a31a7a2e]
+description = "negative value is an error"
+reimplements = "c6c795bf-a288-45e9-86a1-841359ad426d"
diff --git a/exercises/practice/collatz-conjecture/Makefile b/exercises/practice/collatz-conjecture/Makefile
new file mode 100644
index 00000000..fbb5a7de
--- /dev/null
+++ b/exercises/practice/collatz-conjecture/Makefile
@@ -0,0 +1,21 @@
+ERL    := $(shell which erl)
+REBAR3 := $(shell which rebar3)
+
+null  :=
+space := $(null) #
+comma := ,
+
+ifeq ($(ERL),)
+  $(error Can't find Erlang executable 'erl')
+else ifeq ($(REBAR3),)
+  $(error Can't find rebar3)
+endif
+
+compile: ; $(REBAR3) compile
+
+clean: ; $(REBAR3) clean
+
+.PHONY: test
+test:
+	$(REBAR3) eunit \
+	-m $(subst $(space),$(comma),$(basename $(notdir $(wildcard test/*.lfe))))
diff --git a/exercises/practice/collatz-conjecture/rebar.config b/exercises/practice/collatz-conjecture/rebar.config
new file mode 100644
index 00000000..d53487ac
--- /dev/null
+++ b/exercises/practice/collatz-conjecture/rebar.config
@@ -0,0 +1,11 @@
+{plugins, [{rebar3_lfe, "0.4.3"}]}.
+
+{provider_hooks, [{post, [{compile, {lfe, compile}}]}]}.
+
+{deps, [{lfe, "2.1.1"}]}.
+
+{profiles,
+ [{test,
+   [{eunit_compile_opts, [{src_dirs, ["src", "test"]}]},
+    {deps,
+     [{ltest, "0.13.3"}]}]}]}.
diff --git a/exercises/practice/collatz-conjecture/rebar.lock b/exercises/practice/collatz-conjecture/rebar.lock
new file mode 100644
index 00000000..d5a6b3b9
--- /dev/null
+++ b/exercises/practice/collatz-conjecture/rebar.lock
@@ -0,0 +1,8 @@
+{"1.2.0",
+[{<<"lfe">>,{pkg,<<"lfe">>,<<"2.1.1">>},0}]}.
+[
+{pkg_hash,[
+ {<<"lfe">>, <<"4A888B26172D198DC7A5AFEB897E8248AF7D56E1638D9C8249AAF933AE811B96">>}]},
+{pkg_hash_ext,[
+ {<<"lfe">>, <<"C484D3B655D40DED58BC41B17B22F173711C681BF36063A234A9BAA9506947E1">>}]}
+].
diff --git a/exercises/practice/collatz-conjecture/src/collatz-conjecture.app.src b/exercises/practice/collatz-conjecture/src/collatz-conjecture.app.src
new file mode 100644
index 00000000..3d3cfa37
--- /dev/null
+++ b/exercises/practice/collatz-conjecture/src/collatz-conjecture.app.src
@@ -0,0 +1,11 @@
+%% -*- erlang -*-
+{application, 'collatz-conjecture',
+ [{description, ""},
+  {vsn, "0.0.1"},
+  {modules,
+   ['collatz-conjecture']},
+  {registered, []},
+  {applications,
+   [kernel, stdlib]},
+  {included_applications, []},
+  {env, []}]}.
diff --git a/exercises/practice/collatz-conjecture/src/collatz-conjecture.lfe b/exercises/practice/collatz-conjecture/src/collatz-conjecture.lfe
new file mode 100644
index 00000000..47b1b245
--- /dev/null
+++ b/exercises/practice/collatz-conjecture/src/collatz-conjecture.lfe
@@ -0,0 +1,4 @@
+(defmodule collatz-conjecture
+  (export (steps 1)))
+
+ ; Please implement the steps function.
\ No newline at end of file
diff --git a/exercises/practice/collatz-conjecture/test/collatz-conjecture-tests.lfe b/exercises/practice/collatz-conjecture/test/collatz-conjecture-tests.lfe
new file mode 100644
index 00000000..732b5050
--- /dev/null
+++ b/exercises/practice/collatz-conjecture/test/collatz-conjecture-tests.lfe
@@ -0,0 +1,23 @@
+(defmodule collatz-conjecture-tests
+  (behaviour ltest-unit)
+  (export all))
+
+(include-lib "ltest/include/ltest-macros.lfe")
+
+(deftest zero-steps-for-one
+  (is-equal (collatz-conjecture:steps 1) 0))
+
+(deftest divide-if-even
+  (is-equal (collatz-conjecture:steps 16) 4))
+
+(deftest even-and-odd-steps
+  (is-equal (collatz-conjecture:steps 12) 9))
+
+(deftest large-number-of-even-and-odd-steps
+  (is-equal (collatz-conjecture:steps 1000000) 152))
+
+(deftest zero-is-an-error
+  (is-error _ (collatz-conjecture:steps 0)))
+
+(deftest negative-value-is-an-error
+  (is-error _ (collatz-conjecture:steps -1)))