Add ability to adjust merging behavior of configs per fields defined in configurations

CHANGELOG.md

@@ -5,6 +5,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ## [Unreleased]
 
 ### Added
+- Added ability to adjust merging behavior based on field names in configuration. Using `ucfg.FieldMergeValues`, `ucfg.FieldReplaceValues`, `ucfg.FieldAppendValues`, and `ucfg.FieldPrependValues`. #151
 
 ### Changed
 

merge.go

@@ -115,6 +115,10 @@ func mergeConfigDict(opts *options, to, from *Config) Error {
 		}
 
 		old, _ := to.fields.get(k)
+		opts, err := fieldOptsOverride(opts, k, -1)
+		if err != nil {
+			return err
+		}
 		merged, err := mergeValues(opts, old, v)
 		if err != nil {
 			return err
@@ -128,7 +132,12 @@ func mergeConfigDict(opts *options, to, from *Config) Error {
 }
 
 func mergeConfigArr(opts *options, to, from *Config) Error {
-	switch opts.configValueHandling {
+	currHandling := opts.configValueHandling
+	opts, err := fieldOptsOverride(opts, "*", -1)
+	if err != nil {
+		return err
+	}
+	switch currHandling {
 	case cfgReplaceValue:
 		return mergeConfigReplaceArr(opts, to, from)
 
@@ -177,8 +186,13 @@ func mergeConfigMergeArr(opts *options, to, from *Config) Error {
 			field:  fmt.Sprintf("%v", i),
 		}
 
+		// possible for individual index to be replaced
+		idxOpts, err := fieldOptsOverride(opts, "", i)
+		if err != nil {
+			return err
+		}
 		old := to.fields.array()[i]
-		merged, err := mergeValues(opts, old, arr[i])
+		merged, err := mergeValues(idxOpts, old, arr[i])
 		if err != nil {
 			return err
 		}
@@ -515,3 +529,57 @@ func normalizeString(ctx context, opts *options, str string) (value, Error) {
 
 	return newSplice(ctx, opts.meta, varexp), nil
 }
+
+func fieldOptsOverride(opts *options, fieldName string, idx int) (*options, Error) {
+	if opts.fieldHandlingTree == nil {
+		return opts, nil
+	}
+	cfgHandling, child, ok := opts.fieldHandlingTree.fieldHandling(fieldName, idx)
+	child, err := includeWildcard(child, opts.fieldHandlingTree)
+	if err != nil {
+		return nil, err
+	}
+	if !ok {
+		// Only return a new `options` when arriving at new nested child. This
+		// combined with optimizations in `includeWildcard` will ensure that only
+		// a new opts will be created and returned when absolutely required.
+		if child != nil && opts.fieldHandlingTree != child {
+			newOpts := *opts
+			newOpts.fieldHandlingTree = child
+			opts = &newOpts
+		}
+		return opts, nil
+	}
+	// Only return a new `options` if absolutely required.
+	if opts.configValueHandling != cfgHandling || opts.fieldHandlingTree != child {
+		newOpts := *opts
+		newOpts.configValueHandling = cfgHandling
+		newOpts.fieldHandlingTree = child
+		opts = &newOpts
+	}
+	return opts, nil
+}
+
+func includeWildcard(child *fieldHandlingTree, parent *fieldHandlingTree) (*fieldHandlingTree, Error) {
+	if parent == nil {
+		return child, nil
+	}
+	wildcard, err := parent.wildcard()
+	if err != nil {
+		return child, nil
+	}
+	if child == nil && len(parent.fields.dict()) == 1 {
+		// parent is already config with just wildcard
+		return parent, nil
+	}
+	sub := newFieldHandlingTree()
+	if child != nil {
+		if err := sub.merge(child); err != nil {
+			return nil, err.(Error)
+		}
+	}
+	if err := sub.setWildcard(wildcard); err != nil {
+		return nil, err.(Error)
+	}
+	return sub, nil
+}