feat(add): skip re-install when signatures match

- never compare sigs when --pure is used
- avoid re-installing ".yalc/{name}" when sigs match
- improved logic for updating package dependencies

src/add.ts

@@ -87,10 +87,9 @@ export const addPackages = async (
         )
         return null
       }
-      const versionToInstall = version || getLatestPackageVersion(name)
 
+      const versionToInstall = version || getLatestPackageVersion(name)
       const storedPackageDir = getPackageStoreDir(name, versionToInstall)
-
       if (!fs.existsSync(storedPackageDir)) {
         console.log(
           `Could not find package \`${packageName}\` ` + storedPackageDir,
@@ -101,13 +100,10 @@ export const addPackages = async (
 
       const pkg = readPackageManifest(storedPackageDir)
       if (!pkg) {
-        return
+        return null
       }
-      const destYalcCopyDir = join(workingDir, values.yalcPackagesFolder, name)
-
-      emptyDirExcludeNodeModules(destYalcCopyDir)
-      fs.copySync(storedPackageDir, destYalcCopyDir)
 
+      const signature = readSignatureFile(storedPackageDir)
       let replacedVersion = ''
       if (doPure) {
         if (localPkg.workspaces) {
@@ -126,60 +122,65 @@ export const addPackages = async (
         )
       }
       if (!doPure) {
-        const destModulesDir = join(workingDir, 'node_modules', name)
-        fs.removeSync(destModulesDir)
-        if (options.link) {
-          ensureSymlinkSync(destYalcCopyDir, destModulesDir, 'junction')
-        } else {
-          fs.copySync(destYalcCopyDir, destModulesDir)
-        }
-
-        if (!options.noSave) {
+        if (!options.link) {
           const protocol = options.link ? 'link:' : 'file:'
           const localAddress =
             protocol + values.yalcPackagesFolder + '/' + pkg.name
 
           const dependencies = localPkg.dependencies || {}
           const devDependencies = localPkg.devDependencies || {}
-          let whereToAdd = options.dev ? devDependencies : dependencies
 
-          if (options.dev) {
-            if (dependencies[pkg.name]) {
-              replacedVersion = dependencies[pkg.name]
-              delete dependencies[pkg.name]
-            }
-          } else {
-            if (!dependencies[pkg.name]) {
-              if (devDependencies[pkg.name]) {
-                whereToAdd = devDependencies
-              }
-            }
-          }
+          const whereToRemove = devDependencies[pkg.name]
+            ? devDependencies
+            : dependencies
 
-          if (whereToAdd[pkg.name] !== localAddress) {
-            replacedVersion = replacedVersion || whereToAdd[pkg.name]
-            whereToAdd[pkg.name] = localAddress
-            localPkg.dependencies =
-              whereToAdd === dependencies ? dependencies : localPkg.dependencies
-            localPkg.devDependencies =
-              whereToAdd === devDependencies
+          replacedVersion = whereToRemove[pkg.name] || ''
+          if (replacedVersion !== localAddress) {
+            const whereToAdd =
+              options.dev || whereToRemove === devDependencies
                 ? devDependencies
-                : localPkg.devDependencies
+                : dependencies
+
             localPkgUpdated = true
+            whereToAdd[pkg.name] = localAddress
+            if (whereToAdd !== whereToRemove) {
+              delete whereToRemove[pkg.name]
+            }
+          } else {
+            replacedVersion = ''
           }
-          replacedVersion =
-            replacedVersion == localAddress ? '' : replacedVersion
         }
 
+        const destYalcCopyDir = join(
+          workingDir,
+          values.yalcPackagesFolder,
+          name
+        )
+
+        if (signature === readSignatureFile(destYalcCopyDir)) {
+          console.log(
+            `"${packageName}" already exists in the local ".yalc" directory`
+          )
+          return null
+        }
+
+        // Replace the local ".yalc/{name}" directory.
+        fs.removeSync(destYalcCopyDir)
+        fs.copySync(storedPackageDir, destYalcCopyDir)
+
+        // Replace the local "node_modules/{name}" symlink.
+        const nodeModulesPath = join(workingDir, 'node_modules', name)
+        fs.removeSync(nodeModulesPath)
+        ensureSymlinkSync(destYalcCopyDir, nodeModulesPath, 'junction')
+
         const addedAction = options.noSave ? 'linked' : 'added'
         console.log(
           `Package ${pkg.name}@${
             pkg.version
-          } ${addedAction} ==> ${destModulesDir}.`
+          } ${addedAction} ==> ${nodeModulesPath}.`
         )
       }
 
-      const signature = readSignatureFile(storedPackageDir)
       return {
         signature,
         name,