db: convert adjustedOutputLevel into a function

There's benefits to turning adjustedOutputLevel into a function.
1. Multilevel compactions will want to edit the adjustedOutputLevel,
   and currently does so in a hacky way. With this change, I can get
   rid of the hack.
2. Storing the baseLevel is much better because it doesn't change once
   set.

compaction_picker.go

@@ -202,11 +202,9 @@ type pickedCompaction struct {
 	// extraLevels contain additional levels in between the input and output
 	// levels that get compacted in multi level compactions
 	extraLevels []*compactionLevel
-	// adjustedOutputLevel is the output level used for the purpose of
-	// determining the target output file size, overlap bytes, and expanded
-	// bytes, taking into account the base level.
-	adjustedOutputLevel int
-	inputs              []compactionLevel
+	inputs      []compactionLevel
+	// LBase at the time of compaction picking.
+	baseLevel int
 	// L0-specific compaction info. Set to a non-nil value for all compactions
 	// where startLevel == 0 that were generated by L0Sublevels.
 	lcf *manifest.L0CompactionFiles
@@ -246,28 +244,35 @@ func newPickedCompaction(
 			startLevel, baseLevel))
 	}
 
-	adjustedOutputLevel := outputLevel
-	if adjustedOutputLevel > 0 {
-		// Output level is in the range [baseLevel,numLevels]. For the purpose of
-		// determining the target output file size, overlap bytes, and expanded
-		// bytes, we want to adjust the range to [1,numLevels].
-		adjustedOutputLevel = 1 + outputLevel - baseLevel
-	}
-
+	adjustedLevel := adjustedOutputLevel(outputLevel, baseLevel)
 	pc := &pickedCompaction{
 		cmp:                    opts.Comparer.Compare,
 		version:                cur,
+		baseLevel:              baseLevel,
 		inputs:                 []compactionLevel{{level: startLevel}, {level: outputLevel}},
-		adjustedOutputLevel:    adjustedOutputLevel,
-		maxOutputFileSize:      uint64(opts.Level(adjustedOutputLevel).TargetFileSize),
-		maxOverlapBytes:        maxGrandparentOverlapBytes(opts, adjustedOutputLevel),
-		maxReadCompactionBytes: maxReadCompactionBytes(opts, adjustedOutputLevel),
+		maxOutputFileSize:      uint64(opts.Level(adjustedLevel).TargetFileSize),
+		maxOverlapBytes:        maxGrandparentOverlapBytes(opts, adjustedLevel),
+		maxReadCompactionBytes: maxReadCompactionBytes(opts, adjustedLevel),
 	}
 	pc.startLevel = &pc.inputs[0]
 	pc.outputLevel = &pc.inputs[1]
 	return pc
 }
 
+// adjustedOutputLevel is the output level used for the purpose of
+// determining the target output file size, overlap bytes, and expanded
+// bytes, taking into account the base level.
+func adjustedOutputLevel(outputLevel int, baseLevel int) int {
+	adjustedOutputLevel := outputLevel
+	if adjustedOutputLevel > 0 {
+		// Output level is in the range [baseLevel, numLevels]. For the purpose of
+		// determining the target output file size, overlap bytes, and expanded
+		// bytes, we want to adjust the range to [1,numLevels].
+		adjustedOutputLevel = 1 + outputLevel - baseLevel
+	}
+	return adjustedOutputLevel
+}
+
 func newPickedCompactionFromL0(
 	lcf *manifest.L0CompactionFiles, opts *Options, vers *version, baseLevel int, isBase bool,
 ) *pickedCompaction {
@@ -300,7 +305,7 @@ func (pc *pickedCompaction) String() string {
 	var builder strings.Builder
 	builder.WriteString(fmt.Sprintf(`Score=%f, `, pc.score))
 	builder.WriteString(fmt.Sprintf(`Kind=%s, `, pc.kind))
-	builder.WriteString(fmt.Sprintf(`AdjustedOutputLevel=%d, `, pc.adjustedOutputLevel))
+	builder.WriteString(fmt.Sprintf(`AdjustedOutputLevel=%d, `, adjustedOutputLevel(pc.outputLevel.level, pc.baseLevel)))
 	builder.WriteString(fmt.Sprintf(`maxOutputFileSize=%d, `, pc.maxOutputFileSize))
 	builder.WriteString(fmt.Sprintf(`maxReadCompactionBytes=%d, `, pc.maxReadCompactionBytes))
 	builder.WriteString(fmt.Sprintf(`smallest=%s, `, pc.smallest))
@@ -324,7 +329,7 @@ func (pc *pickedCompaction) clone() *pickedCompaction {
 		cmp:                    pc.cmp,
 		score:                  pc.score,
 		kind:                   pc.kind,
-		adjustedOutputLevel:    pc.adjustedOutputLevel,
+		baseLevel:              pc.baseLevel,
 		maxOutputFileSize:      pc.maxOutputFileSize,
 		maxOverlapBytes:        pc.maxOverlapBytes,
 		maxReadCompactionBytes: pc.maxReadCompactionBytes,
@@ -402,7 +407,7 @@ func (pc *pickedCompaction) setupInputs(
 	// growing a compaction results in a larger size, the original compaction
 	// is used instead.
 	maxExpandedBytes := expandedCompactionByteSizeLimit(
-		opts, pc.adjustedOutputLevel, diskAvailBytes,
+		opts, adjustedOutputLevel(pc.outputLevel.level, pc.baseLevel), diskAvailBytes,
 	)
 
 	// Expand the initial inputs to a clean cut.
@@ -560,8 +565,6 @@ func (pc *pickedCompaction) setupMultiLevelCandidate(opts *Options, diskAvailByt
 	pc.startLevel = &pc.inputs[0]
 	pc.extraLevels = []*compactionLevel{&pc.inputs[1]}
 	pc.outputLevel = &pc.inputs[2]
-
-	pc.adjustedOutputLevel++
 	return pc.setupInputs(opts, diskAvailBytes, pc.extraLevels[len(pc.extraLevels)-1])
 }
 
@@ -1699,7 +1702,7 @@ func (pc *pickedCompaction) maybeAddLevel(opts *Options, diskAvailBytes uint64)
 		return pc
 	}
 	if pc.compactionSize() > expandedCompactionByteSizeLimit(
-		opts, pc.adjustedOutputLevel, diskAvailBytes) {
+		opts, adjustedOutputLevel(pc.outputLevel.level, pc.baseLevel), diskAvailBytes) {
 		// Don't add a level if the current compaction exceeds the compaction size limit
 		return pc
 	}