Add DumpN() functions

cdb.go

@@ -74,6 +74,45 @@ func (c *Cdb) Data(key []byte) (data []byte, err error) {
 	return
 }
 
+// DumpN returns n records, starting at record number o, from the cdb as an
+// array of [2][]byte values: key, data.
+// If n is 0, then all of the records are returned.
+func (c *Cdb) DumpN(n uint64, o uint64) (recs [][2][]byte) {
+	intBuf := make([]byte, 4)
+	pos := uint32(0)
+	readNum := func () uint32 {
+		c.r.ReadAt(intBuf, int64(pos))
+		pos += 4
+		return binary.LittleEndian.Uint32(intBuf)
+	}
+
+	eod := readNum()
+	pos = headerSize  // skip header
+	recNum := uint64(0)
+	for pos < eod && (n == 0 || recNum < n) {
+		// CDB records are laid out as: [klen (uint32)] [dlen (uint32)] [key (klen bytes)] [data (dlen bytes)]
+		klen := readNum()
+		dlen := readNum()
+		if recNum >= o {
+			// read key and data:
+			k := make([]byte, klen)
+			d := make([]byte, dlen)
+			c.r.ReadAt(k, int64(pos))
+			pos += klen
+			c.r.ReadAt(d, int64(pos))
+			pos += dlen
+			recs = append(recs, [2][]byte{k, d})
+		} else {
+			// skip:
+			pos += klen + dlen
+		}
+
+		recNum++
+	}
+
+	return
+}
+
 // FindStart resets the cdb to search for the first record under a new key.
 func (c *Cdb) FindStart() { c.loop = 0 }
 

dump.go

@@ -7,11 +7,21 @@ import (
 	"io"
 )
 
-// Dump reads the cdb-formatted data in r and dumps it as a series of formatted
-// records (+klen,dlen:key->data\n) and a final newline to w.
+// Dump reads records from the cdb-formatted data in r and dumps it as a
+// series of formatted records (+klen,dlen:key->data\n) and a final newline to w.
 // The output of Dump is suitable as input to Make.
 // See http://cr.yp.to/cdb/cdbmake.html for details on the record format.
-func Dump(w io.Writer, r io.Reader) (err error) {
+func Dump(w io.Writer, r io.Reader) error {
+	return DumpN(0, 0, w, r)
+}
+
+// DumpN reads n records, starting at record number o,  from the cdb-formatted
+// data in r and dumps it as a series of formatted records
+// (+klen,dlen:key->data\n) and a final newline to w.
+// If n is 0, then all of the records are dumped.
+// The output of DumpN is suitable as input to Make.
+// See http://cr.yp.to/cdb/cdbmake.html for details on the record format.
+func DumpN(n uint64, o uint64, w io.Writer, r io.Reader) (err error) {
 	defer func() { // Centralize exception handling.
 		if e := recover(); e != nil {
 			err = e.(error)
@@ -29,14 +39,18 @@ func Dump(w io.Writer, r io.Reader) (err error) {
 	}
 
 	pos := headerSize
-	for pos < eod {
+	recNum := uint64(0)
+	for pos < eod && (n == 0 || recNum < n) {
 		klen, dlen := readNum(), readNum()
-		rw.writeString(fmt.Sprintf("+%d,%d:", klen, dlen))
-		rw.copyn(rb, klen)
-		rw.writeString("->")
-		rw.copyn(rb, dlen)
-		rw.writeString("\n")
+		if recNum >= o {
+			rw.writeString(fmt.Sprintf("+%d,%d:", klen, dlen))
+			rw.copyn(rb, klen)
+			rw.writeString("->")
+			rw.copyn(rb, dlen)
+			rw.writeString("\n")
+		}
 		pos += 8 + klen + dlen
+		recNum++
 	}
 	rw.writeString("\n")