feat(tags/derive): add semver tag derivation functionality (#79)

src/derive.ts

@@ -0,0 +1,105 @@
+import { parse } from 'csv-parse/sync'
+import Handlebars from 'handlebars'
+import * as semver from 'semver'
+
+/**
+ * Context containing parsed semver components for template rendering.
+ */
+export interface SemverContext {
+  /** "v" or "V" if input had a prefix, empty string otherwise */
+  prefix: string
+  /** Major version number */
+  major: number
+  /** Minor version number */
+  minor: number
+  /** Patch version number */
+  patch: number
+  /** Prerelease identifier (e.g., "beta.1"), empty if none */
+  prerelease: string
+  /** Build metadata (e.g., "build.123"), empty if none */
+  build: string
+  /** Full version string without prefix */
+  version: string
+}
+
+/**
+ * Parse a version string into semver components.
+ *
+ * @param input - Version string (e.g., "v1.2.3", "1.2.3-beta.1+build.456")
+ * @returns Parsed semver context
+ * @throws Error if the version string is not valid semver
+ */
+export function parseSemver(input: string): SemverContext {
+  const trimmed = input.trim()
+  if (!trimmed) {
+    throw new Error('Invalid semver: empty string')
+  }
+
+  // Check for v/V prefix and preserve original case
+  const firstChar = trimmed[0]
+  const hasPrefix = firstChar === 'v' || firstChar === 'V'
+  const prefix = hasPrefix ? firstChar : ''
+  const versionStr = hasPrefix ? trimmed.slice(1) : trimmed
+
+  // Parse with semver library
+  const parsed = semver.parse(versionStr)
+  if (!parsed) {
+    throw new Error(`Invalid semver: '${input}'`)
+  }
+
+  return {
+    prefix,
+    major: parsed.major,
+    minor: parsed.minor,
+    patch: parsed.patch,
+    prerelease: parsed.prerelease.join('.'),
+    build: parsed.build.join('.'),
+    version: parsed.version
+  }
+}
+
+/**
+ * Render a single template string with semver context using Handlebars.
+ * Supports {{#if variable}}...{{/if}} conditionals for optional sections.
+ *
+ * @param template - Handlebars template string
+ * @param ctx - Semver context for substitution
+ * @returns Rendered string
+ */
+export function renderTemplate(template: string, ctx: SemverContext): string {
+  const compiled = Handlebars.compile(template, { noEscape: true })
+  // Convert numbers to strings so 0 is truthy in conditionals
+  const stringCtx = {
+    ...ctx,
+    major: String(ctx.major),
+    minor: String(ctx.minor),
+    patch: String(ctx.patch)
+  }
+  return compiled(stringCtx)
+}
+
+/**
+ * Derive tags from a semver version string using a template.
+ *
+ * @param deriveFrom - Semver version string (e.g., "v1.2.3")
+ * @param template - CSV/newline-delimited Handlebars template string
+ * @returns Array of derived tag strings
+ */
+export function deriveTags(deriveFrom: string, template: string): string[] {
+  const ctx = parseSemver(deriveFrom)
+
+  // Render template with Handlebars first, enabling conditional tag inclusion
+  const rendered = renderTemplate(template, ctx)
+
+  // Parse rendered result as CSV/newline delimited
+  const tags = (
+    parse(rendered, {
+      delimiter: ',',
+      trim: true,
+      relax_column_count: true
+    }) as string[][]
+  ).flat()
+
+  // Exclude empty tags and tags that are just the prefix with no version data
+  return tags.filter((tag) => tag.length > 0 && tag !== ctx.prefix)
+}

src/inputs.ts

@@ -1,6 +1,7 @@
 import * as core from '@actions/core'
 import * as github from '@actions/github'
 import { parse } from 'csv-parse/sync'
+import { deriveTags } from './derive.js'
 
 const WHEN_EXISTS_MODES = ['update', 'skip', 'fail'] as const
 export type WhenExistsMode = (typeof WHEN_EXISTS_MODES)[number]
@@ -33,13 +34,19 @@ function validateWhenExists(input: string): WhenExistsMode {
   return input as WhenExistsMode
 }
 
+const DEFAULT_DERIVE_FROM_TEMPLATE =
+  '{{prefix}}{{major}},{{prefix}}{{major}}.{{minor}}'
+
 /**
  * Get and validate action inputs.
  *
  * @returns Parsed and validated inputs
  */
 export function getInputs(): Inputs {
-  const tagsInput: string = core.getInput('tags', { required: true })
+  const tagsInput: string = core.getInput('tags')
+  const deriveFrom: string = core.getInput('derive_from')
+  const deriveFromTemplate: string =
+    core.getInput('derive_from_template') || DEFAULT_DERIVE_FROM_TEMPLATE
   const defaultRef: string = core.getInput('ref')
   const whenExistsInput = core.getInput('when_exists') || 'update'
   const whenExists = validateWhenExists(whenExistsInput)
@@ -51,14 +58,31 @@ export function getInputs(): Inputs {
 
   const { owner, repo } = github.context.repo
 
-  // Parse tags as CSV/newline delimited strings
-  const tags = (
-    parse(tagsInput, {
-      delimiter: ',',
-      trim: true,
-      relax_column_count: true
-    }) as string[][]
-  ).flat()
+  // Parse explicit tags as CSV/newline delimited strings
+  const explicitTags: string[] = tagsInput
+    ? (
+        parse(tagsInput, {
+          delimiter: ',',
+          trim: true,
+          relax_column_count: true
+        }) as string[][]
+      ).flat()
+    : []
+
+  // Derive tags from semver version string if provided
+  const derivedTags: string[] = deriveFrom
+    ? deriveTags(deriveFrom, deriveFromTemplate)
+    : []
+
+  // Combine explicit and derived tags
+  const tags = [...explicitTags, ...derivedTags]
+
+  // Validate that at least one tag source is provided
+  if (tags.length === 0) {
+    throw new Error(
+      "No tags specified. Provide 'tags' input, 'derive_from' input, or both."
+    )
+  }
 
   return {
     tags,