package jsn import ( "bytes" "github.com/cespare/xxhash/v2" ) func Filter(w *bytes.Buffer, b []byte, keys []string) error { var err error kmap := make(map[uint64]struct{}, len(keys)) for i := range keys { kmap[xxhash.Sum64String(keys[i])] = struct{}{} } // is an list isList := false // list item item := 0 // field in an object field := 0 s, e, d := 0, 0, 0 var k []byte state := expectKey for i := 0; i < len(b); i++ { if state == expectObjClose || state == expectListClose { switch b[i] { case '{', '[': d++ case '}', ']': d-- } } if state == expectKey { switch b[i] { case '[': if !isList { err = w.WriteByte('[') } isList = true case '{': if item == 0 { err = w.WriteByte('{') } else { _, err = w.Write([]byte("},{")) } field = 0 item++ } if err != nil { return err } } switch { case state == expectKey && b[i] == '"': state = expectKeyClose s = i case state == expectKeyClose && b[i] == '"': state = expectColon k = b[(s + 1):i] case state == expectColon && b[i] == ':': state = expectValue case state == expectValue && b[i] == '"': state = expectString case state == expectString && b[i] == '"': e = i case state == expectValue && b[i] == '[': state = expectListClose d++ case state == expectListClose && d == 0 && b[i] == ']': e = i case state == expectValue && b[i] == '{': state = expectObjClose d++ case state == expectObjClose && d == 0 && b[i] == '}': e = i case state == expectValue && (b[i] >= '0' && b[i] <= '9'): state = expectNumClose case state == expectNumClose && ((b[i] < '0' || b[i] > '9') && (b[i] != '.' && b[i] != 'e' && b[i] != 'E' && b[i] != '+' && b[i] != '-')): e = i - 1 case state == expectValue && (b[i] == 'f' || b[i] == 'F' || b[i] == 't' || b[i] == 'T'): state = expectBoolClose case state == expectBoolClose && (b[i] == 'e' || b[i] == 'E'): e = i case state == expectValue && b[i] == 'n': state = expectNull case state == expectNull && b[i] == 'l': e = i } if e != 0 { state = expectKey cb := b[s:(e + 1)] e = 0 if _, ok := kmap[xxhash.Sum64(k)]; !ok { continue } if field != 0 { if err := w.WriteByte(','); err != nil { return err } } sk := 0 for i := 0; i < len(cb); i++ { if cb[i] == '\n' || cb[i] == '\t' { if _, err := w.Write(cb[sk:i]); err != nil { return err } sk = i + 1 } } if sk > 0 && sk < len(cb) { _, err = w.Write(cb[sk:]) } else { _, err = w.Write(cb) } if err != nil { return err } field++ } } if item != 0 { if err := w.WriteByte('}'); err != nil { return err } } if isList { if err := w.WriteByte(']'); err != nil { return err } } return nil }