Added documentation for version_compare()

Ticket: CFE-3991
Signed-off-by: Ole Herman Schumacher Elgesem <[email protected]>

reference/functions/cf_version_after.markdown

@@ -18,7 +18,7 @@ Output:
 
 [%CFEngine_include_snippet(cf_version_after.cf, #\+begin_src\s+example_output\s*, .*end_src)%]
 
-**See also:** `cf_version_maximum()`, `cf_version_minimum()`, `cf_version_before()`, `cf_version_at()`, `cf_version_between()`.
+**See also:** `version_compare()`, `cf_version_maximum()`, `cf_version_minimum()`, `cf_version_before()`, `cf_version_at()`, `cf_version_between()`.
 
 **History:**
 

reference/functions/cf_version_at.markdown

@@ -18,7 +18,7 @@ Output:
 
 [%CFEngine_include_snippet(cf_version_at.cf, #\+begin_src\s+example_output\s*, .*end_src)%]
 
-**See also:** `cf_version_maximum()`, `cf_version_minimum()`, `cf_version_after()`, `cf_version_before()`, `cf_version_between()`.
+**See also:** `version_compare()`, `cf_version_maximum()`, `cf_version_minimum()`, `cf_version_after()`, `cf_version_before()`, `cf_version_between()`.
 
 **History:**
 

reference/functions/cf_version_before.markdown

@@ -18,7 +18,7 @@ Output:
 
 [%CFEngine_include_snippet(cf_version_before.cf, #\+begin_src\s+example_output\s*, .*end_src)%]
 
-**See also:** `cf_version_maximum()`, `cf_version_minimum()`, `cf_version_after()`, `cf_version_at()`, `cf_version_between()`.
+**See also:** `version_compare()`, `cf_version_maximum()`, `cf_version_minimum()`, `cf_version_after()`, `cf_version_at()`, `cf_version_between()`.
 
 **History:**
 

reference/functions/cf_version_between.markdown

@@ -18,7 +18,7 @@ Output:
 
 [%CFEngine_include_snippet(cf_version_between.cf, #\+begin_src\s+example_output\s*, .*end_src)%]
 
-**See also:** `cf_version_maximum()`, `cf_version_minimum()`, `cf_version_after()`, `cf_version_before()`, `cf_version_at()`.
+**See also:** `version_compare()`, `cf_version_maximum()`, `cf_version_minimum()`, `cf_version_after()`, `cf_version_before()`, `cf_version_at()`.
 
 **History:**
 

reference/functions/cf_version_maximum.markdown

@@ -18,7 +18,7 @@ Output:
 
 [%CFEngine_include_snippet(cf_version_maximum.cf, #\+begin_src\s+example_output\s*, .*end_src)%]
 
-**See also:** `cf_version_minimum()`, `cf_version_after()`, `cf_version_before()`, `cf_version_at()`, `cf_version_between()`.
+**See also:** `version_compare()`, `cf_version_minimum()`, `cf_version_after()`, `cf_version_before()`, `cf_version_at()`, `cf_version_between()`.
 
 **History:**
 

reference/functions/cf_version_minimum.markdown

@@ -18,7 +18,7 @@ Output:
 
 [%CFEngine_include_snippet(cf_version_minimum.cf, #\+begin_src\s+example_output\s*, .*end_src)%]
 
-**See also:** `cf_version_maximum()`, `cf_version_after()`, `cf_version_before()`, `cf_version_at()`, `cf_version_between()`.
+**See also:** `version_compare()`, `cf_version_maximum()`, `cf_version_after()`, `cf_version_before()`, `cf_version_at()`, `cf_version_between()`.
 
 **History:**
 

reference/functions/version_compare.markdown

@@ -0,0 +1,83 @@
+---
+layout: default
+title: version_compare
+published: true
+---
+
+[%CFEngine_function_prototype(string)%]
+
+**Description:** Returns `true` if the specified version comparison expression is true.
+
+[%CFEngine_function_attributes(string)%]
+
+The `version_compare()` function can be used to compare 2 arbitrary semver version numbers.
+This can be useful if you have 2 versions of a package and you want to know if they are the same version, if one is newer than the other etc.
+
+**Example:**
+
+[%CFEngine_include_snippet(version_compare.cf, #\+begin_src cfengine3, .*end_src)%]
+
+Output:
+
+[%CFEngine_include_snippet(version_compare.cf, #\+begin_src\s+example_output\s*, .*end_src)%]
+
+**Notes:**
+
+Internally, `version_compare` uses the same version comparison logic as other version functions (`cf_version_minimum`, etc.).
+This means that only the **`major.minor.patch`** parts of the version string are compared, everything after patch is ignored so `1.2.3`, `1.2.3-1`, `1.2.3-2`, `1.2.3+git1234`, `1.2.3a` are all considered the same version.
+
+In policy you can combine `version_compare()` and `strcmp()` to get more info if desirable:
+
+```cfengine3
+bundle agent main
+{
+  reports:
+    "Same patch version, but not exactly the same version string"
+      if => and(
+        version_compare("$(a)", "=", "$(b)"),
+        not(strcmp("$(a)", "$(b)"))
+      );
+}
+```
+
+CFEngine's version comparison functions also support partial version numbers, so if you supply only a major version, or a major and minor version, but no patch version, only the provided parts are compared:
+
+```cfengine3
+bundle agent main
+{
+  vars:
+    "patch_a"
+      string => "3.21.1";
+    "patch_b"
+      string => "4.0.1";
+
+  reports:
+    "patch_a is a part of the 3.21 series: 3.21 == $(patch_a)"
+      if => version_compare("3.21", "==", "$(patch_a)");
+    "patch_b is in major version 4: 4 == $(patch_b)"
+      if => version_compare("4", "==", "$(patch_b)");
+}
+```
+
+Beware that using partial version numbers can lead to situations with surprising results, the `>`, `<`, `=`, `!=` operators may not behave exactly as the policy writer expects:
+
+```cfengine3
+bundle agent main
+{
+  reports:
+    "3.22.1 > 3.22" # No, won't be printed
+      if => version_compare("3.22.1", ">", "3.22");
+    "3.22.1 >= 3.22" # Yes, will be printed
+      if => version_compare("3.22.1", ">=", "3.22");
+}
+```
+
+Since `3.22` is less specific than `3.22.1` they are considered equal - all of the parts which can be compared are the same.
+For the version comparison logic, if you are asking if something is greater than `3.22`, it must be at least `3.23`, `3.22.1` is not enough.
+Thus, it is often more intuitive to use the `>=` operator to mean all versions at or after the specified version (regardless of how specific the version numbers are).
+
+**See also:** `cf_version_minimum()`, `cf_version_maximum()`, `cf_version_after()`, `cf_version_before()`, `cf_version_at()`, `cf_version_between()`.
+
+**History:**
+
+* Introduced in 3.23.0