databricks-cli/libs/dyn/merge/merge.go

118 lines
3.6 KiB
Go

package merge
import (
"fmt"
"github.com/databricks/cli/libs/dyn"
)
// Merge recursively merges the specified values.
//
// Semantics are as follows:
// * Merging x with nil or nil with x always yields x.
// * Merging maps a and b means entries from map b take precedence.
// * Merging sequences a and b means concatenating them.
//
// Merging retains and accumulates the locations metadata associated with the values.
// This allows users of the module to track the provenance of values across merging of
// configuration trees, which is useful for reporting errors and warnings.
//
// Semantics for location metadata in the merged value are similar to the semantics
// for the values themselves:
//
// - When merging x with nil or nil with x, the location of x is retained.
//
// - When merging maps or sequences, the combined value retains the location of a and
// accumulates the location of b. The individual elements of the map or sequence retain
// their original locations, i.e., whether they were originally defined in a or b.
//
// The rationale for retaining location of a is that we would like to return
// the first location a bit of configuration showed up when reporting errors and warnings.
//
// - Merging primitive values means using the incoming value `b`. The location of the
// incoming value is retained and the location of the existing value `a` is accumulated.
// This is because the incoming value overwrites the existing value.
func Merge(a, b dyn.Value) (dyn.Value, error) {
return merge(a, b)
}
func merge(a, b dyn.Value) (dyn.Value, error) {
ak := a.Kind()
bk := b.Kind()
// If a is nil, return b.
if ak == dyn.KindNil {
return b.AppendLocationsFromValue(a), nil
}
// If b is nil, return a.
if bk == dyn.KindNil {
return a.AppendLocationsFromValue(b), nil
}
// Call the appropriate merge function based on the kind of a and b.
switch ak {
case dyn.KindMap:
if bk != dyn.KindMap {
return dyn.InvalidValue, fmt.Errorf("cannot merge map with %s", bk)
}
return mergeMap(a, b)
case dyn.KindSequence:
if bk != dyn.KindSequence {
return dyn.InvalidValue, fmt.Errorf("cannot merge sequence with %s", bk)
}
return mergeSequence(a, b)
default:
if ak != bk {
return dyn.InvalidValue, fmt.Errorf("cannot merge %s with %s", ak, bk)
}
return mergePrimitive(a, b)
}
}
func mergeMap(a, b dyn.Value) (dyn.Value, error) {
out := dyn.NewMapping()
am := a.MustMap()
bm := b.MustMap()
// Add the values from a into the output map.
out.Merge(am)
// Merge the values from b into the output map.
for _, pair := range bm.Pairs() {
pk := pair.Key
pv := pair.Value
if ov, ok := out.Get(pk); ok {
// If the key already exists, merge the values.
merged, err := merge(ov, pv)
if err != nil {
return dyn.InvalidValue, err
}
out.Set(pk, merged)
} else {
// Otherwise, just set the value.
out.Set(pk, pv)
}
}
// Preserve the location of the first value. Accumulate the locations of the second value.
return dyn.NewValue(out, a.Locations()).AppendLocationsFromValue(b), nil
}
func mergeSequence(a, b dyn.Value) (dyn.Value, error) {
as := a.MustSequence()
bs := b.MustSequence()
// Merging sequences means concatenating them.
out := make([]dyn.Value, len(as)+len(bs))
copy(out[:], as)
copy(out[len(as):], bs)
// Preserve the location of the first value. Accumulate the locations of the second value.
return dyn.NewValue(out, a.Locations()).AppendLocationsFromValue(b), nil
}
func mergePrimitive(a, b dyn.Value) (dyn.Value, error) {
// Merging primitive values means using the incoming value.
return b.AppendLocationsFromValue(a), nil
}