Derive version from git (hamcrest#419)

Instead of hard-coding the version into build.gradle, this change derives the version from git tags.

If the HEAD commit has a tag in the format "`vM.N`", the version number `M.N` is used directly. Otherwise, a snapshot version is assumed, and the version becomes `M.(N+1)-SNAPSHOT`.

.github/workflows/build.yml

@@ -21,6 +21,8 @@ jobs:
             experimental: true
     steps:
       - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
       - name: Set up JDK ${{ matrix.java }}
         uses: actions/setup-java@v4
         with:

build.gradle

@@ -2,9 +2,19 @@ plugins {
     id "signing"
     id "maven-publish"
 }
+apply from: "gradle/versioning.gradle"
 
 group = "org.hamcrest"
-version = "3.1-SNAPSHOT"
+version = getMavenVersion()
+
+tasks.register("showVersion") {
+    group = "Build"
+    description = "Show the version, as derived from git tags"
+    doLast {
+        println("git version: " + getGitVersion())
+        println("mvn version: " + getMavenVersion())
+    }
+}
 
 subprojects {
     apply plugin: 'checkstyle'

gradle/versioning.gradle

@@ -0,0 +1,25 @@
+ext.getGitVersion = { ->
+    def stdout = new ByteArrayOutputStream()
+    exec {
+        commandLine "git", "describe", "--tags"
+        standardOutput = stdout
+    }
+    return stdout.toString().trim().replaceAll("v", "")
+}
+
+ext.getMavenVersion = { ->
+    def gitVersion = getGitVersion()
+    if (!gitVersion.contains("-")) {
+        // We're directly on a tagged commit
+        return gitVersion
+    }
+
+    def match = gitVersion =~ /^(\d+)\.(\d+)-.*$/
+    // Make sure we're only using the major.minor version.
+    assert match : "Unexpected version from git: ${gitVersion}"
+
+    // We're not directly on a tagged commit, so increment the minor version and call it a snapshot
+    def (_, major, minor) = match[0]
+    def mvnMinor = minor.toInteger() + 1
+    return "${major}.${mvnMinor}-SNAPSHOT"
+}