go/zodb/btree: Introduce constants for min/max key value

We were already using math.Min<Key> in one place, but the number of such
places is going to increase. -> Keep min/max definition in only one place.

go/zodb/btree/btree.go.in

@@ -102,6 +102,9 @@ type BucketEntry struct {
 	value interface{}
 }
 
+const _KeyMin KEY = math.Min<Key>
+const _KeyMax KEY = math.Max<Key>
+
 // ---- access []entry ----
 
 // Key returns BTree entry key.
@@ -583,7 +586,7 @@ func (bt *btreeState) PySetState(pystate interface{}) (err error) {
 	var kprev int64
 	var childrenKind int // 1 - BTree, 2 - Bucket
 	for i, idx := 0, 0; i < n; i++ {
-		key := int64(math.Min<Key>) // KEY(-∞)   (qualifies for ≤)
+		key := int64(_KeyMin) // KEY(-∞)   (qualifies for ≤)
 		if i > 0 {
 			// key[0] is unused and not saved
 			key, ok = t[idx].(int64) // XXX Xint

go/zodb/btree/gen-btree

@@ -28,6 +28,8 @@ out=$3
 kind=${KIND,,}	# IO -> io
 Key=${KEY^}
 
+KEYKIND=${KIND:0:1}	# IO -> I
+
 input=$(dirname $0)/btree.go.in
 
 echo "// Code generated by gen-btree; DO NOT EDIT." >$out
@@ -45,4 +47,6 @@ sed	\
 	-e "s/\bBucketEntry\b/${KIND}BucketEntry/g"	\
 	-e "s/\bbtreeState\b/${kind}btreeState/g"	\
 	-e "s/\bbucketState\b/${kind}bucketState/g"	\
+	-e "s/\b_KeyMin\b/_${KEYKIND}KeyMin/g"		\
+	-e "s/\b_KeyMax\b/_${KEYKIND}KeyMax/g"		\
 	$input >>$out

go/zodb/btree/ziobtree.go

@@ -104,6 +104,9 @@ type IOBucketEntry struct {
 	value interface{}
 }
 
+const _IKeyMin int32 = math.MinInt32
+const _IKeyMax int32 = math.MaxInt32
+
 // ---- access []entry ----
 
 // Key returns IOBTree entry key.
@@ -585,7 +588,7 @@ func (bt *iobtreeState) PySetState(pystate interface{}) (err error) {
 	var kprev int64
 	var childrenKind int // 1 - IOBTree, 2 - IOBucket
 	for i, idx := 0, 0; i < n; i++ {
-		key := int64(math.MinInt32) // int32(-∞)   (qualifies for ≤)
+		key := int64(_IKeyMin) // int32(-∞)   (qualifies for ≤)
 		if i > 0 {
 			// key[0] is unused and not saved
 			key, ok = t[idx].(int64) // XXX Xint

go/zodb/btree/zlobtree.go

@@ -104,6 +104,9 @@ type LOBucketEntry struct {
 	value interface{}
 }
 
+const _LKeyMin int64 = math.MinInt64
+const _LKeyMax int64 = math.MaxInt64
+
 // ---- access []entry ----
 
 // Key returns LOBTree entry key.
@@ -585,7 +588,7 @@ func (bt *lobtreeState) PySetState(pystate interface{}) (err error) {
 	var kprev int64
 	var childrenKind int // 1 - LOBTree, 2 - LOBucket
 	for i, idx := 0, 0; i < n; i++ {
-		key := int64(math.MinInt64) // int64(-∞)   (qualifies for ≤)
+		key := int64(_LKeyMin) // int64(-∞)   (qualifies for ≤)
 		if i > 0 {
 			// key[0] is unused and not saved
 			key, ok = t[idx].(int64) // XXX Xint