[GitHub] [arrow] emkornfield commented on a change in pull request #10379: ARROW-12851: [Go][Parquet] Add Golang Parquet encoding package

[GitBox <gi...@apache.org>]

emkornfield commented on a change in pull request #10379:
URL: https://github.com/apache/arrow/pull/10379#discussion_r644087064



##########
File path: go/parquet/internal/encoding/boolean_decoder.go
##########
@@ -0,0 +1,98 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package encoding
+
+import (
+	"github.com/apache/arrow/go/arrow/bitutil"
+	"github.com/apache/arrow/go/parquet"
+	"github.com/apache/arrow/go/parquet/internal/utils"
+)
+
+// PlainBooleanDecoder is for the Plain Encoding type, there is no
+// dictionary decoding for bools.
+type PlainBooleanDecoder struct {
+	decoder
+
+	bitOffset int
+}
+
+// Type for the PlainBooleanDecoder is parquet.Types.Boolean
+func (PlainBooleanDecoder) Type() parquet.Type {
+	return parquet.Types.Boolean
+}
+
+// Decode fills out with bools decoded from the data at the current point
+// or until we reach the end of the data.
+//
+// Returns the number of values decoded
+func (dec *PlainBooleanDecoder) Decode(out []bool) (int, error) {
+	max := utils.MinInt(len(out), dec.nvals)
+
+	// if we aren't at a byte boundary, then get bools until we hit
+	// a byte boundary with the bit offset.
+	i := 0
+	for dec.bitOffset != 0 && dec.bitOffset < 8 && i < max {
+		out[i] = (dec.data[0] & byte(1<<dec.bitOffset)) != 0
+		dec.bitOffset++
+		i++
+	}
+	if dec.bitOffset == 8 {
+		dec.bitOffset = 0
+	}
+
+	// determine the number of full bytes worth of bits we can decode
+	// given the number of values we want to decode.
+	bitsRemain := max - i
+	batch := bitsRemain / 8 * 8
+	if batch > 0 { // only go in here if there's at least one full byte to decode
+		if i > 0 { // skip our data forward if we decoded anything above
+			dec.data = dec.data[1:]
+			out = out[i:]
+		}
+		// determine the number of aligned bytes we can grab using SIMD optimized
+		// functions to improve performance.
+		alignedBytes := bitutil.BytesForBits(int64(batch))
+		utils.BytesToBools(dec.data[:alignedBytes], out)
+		dec.data = dec.data[alignedBytes:]
+		out = out[alignedBytes*8:]
+	}
+
+	// grab any trailing bits now that we've got our aligned bytes.
+	for ; dec.bitOffset < (bitsRemain - batch); dec.bitOffset++ {
+		out[dec.bitOffset] = (dec.data[0] & byte(1<<dec.bitOffset)) != 0

Review comment:
       might be nice factor out the unaligned bit extract?




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