// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// This file contains the implementation of Go's map type.
//
// The map is just a hash table. The data is arranged
// into an array of buckets. Each bucket contains up to
// 8 key/value pairs. The low-order bits of the hash are
// used to select a bucket. Each bucket contains a few
// high-order bits of each hash to distinguish the entries
// within a single bucket.
//
// If more than 8 keys hash to a bucket, we chain on
// extra buckets.
//
// When the hashtable grows, we allocate a new array
// of buckets twice as big. Buckets are incrementally
// copied from the old bucket array to the new bucket array.
//
// Map iterators walk through the array of buckets and
// return the keys in walk order (bucket #, then overflow
// chain order, then bucket index). To maintain iteration
// semantics, we never move keys within their bucket (if
// we did, keys might be returned 0 or 2 times). When
// growing the table, iterators remain iterating through the
// old table and must check the new table if the bucket
// they are iterating through has been moved ("evacuated")
// to the new table.
// Maximum number of key/value pairs a bucket can hold.
#define BUCKETSIZE 8
// Maximum average load of a bucket that triggers growth.
#define LOAD 6.5
// Picking LOAD: too large and we have lots of overflow
// buckets, too small and we waste a lot of space. I wrote
// a simple program to check some stats for different loads:
// (64-bit, 8 byte keys and values)
// LOAD %overflow bytes/entry hitprobe missprobe
// 4.00 2.13 20.77 3.00 4.00
// 4.50 4.05 17.30 3.25 4.50
// 5.00 6.85 14.77 3.50 5.00
// 5.50 10.55 12.94 3.75 5.50
// 6.00 15.27 11.67 4.00 6.00
// 6.50 20.90 10.79 4.25 6.50
// 7.00 27.14 10.15 4.50 7.00
// 7.50 34.03 9.73 4.75 7.50
// 8.00 41.10 9.40 5.00 8.00
//
// %overflow = percentage of buckets which have an overflow bucket
// bytes/entry = overhead bytes used per key/value pair
// hitprobe = # of entries to check when looking up a present key
// missprobe = # of entries to check when looking up an absent key
//
// Keep in mind this data is for maximally loaded tables, i.e. just
// before the table grows. Typical tables will be somewhat less loaded.
// Maximum key or value size to keep inline (instead of mallocing per element).
// Must fit in a uint8.
// Fast versions cannot handle big values - the cutoff size for
// fast versions in ../../cmd/gc/walk.c must be at most this value.
#define MAXKEYSIZE 128
#define MAXVALUESIZE 128
typedef struct Bucket Bucket;
struct Bucket
{
// Note: the format of the Bucket is encoded in ../../cmd/gc/reflect.c and
// ../reflect/type.go. Don't change this structure without also changing that code!
uint8 tophash[BUCKETSIZE]; // top 8 bits of hash of each entry (or special mark below)
Bucket *overflow; // overflow bucket, if any
uint64 data[1]; // BUCKETSIZE keys followed by BUCKETSIZE values
};
// NOTE: packing all the keys together and then all the values together makes the
// code a bit more complicated than alternating key/value/key/value/... but it allows
// us to eliminate padding which would be needed for, e.g., map[int64]int8.
// tophash values. We reserve a few possibilities for special marks.
// Each bucket (including its overflow buckets, if any) will have either all or none of its
// entries in the Evacuated* states (except during the evacuate() method, which only happens
// during map writes and thus no one else can observe the map during that time).
enum
{
Empty = 0, // cell is empty
EvacuatedEmpty = 1, // cell is empty, bucket is evacuated.
EvacuatedX = 2, // key/value is valid. Entry has been evacuated to first half of larger table.
EvacuatedY = 3, // same as above, but evacuated to second half of larger table.
MinTopHash = 4, // minimum tophash for a normal filled cell.
};
#define evacuated(b) ((b)->tophash[0] > Empty && (b)->tophash[0] < MinTopHash)
struct Hmap
{
// Note: the format of the Hmap is encoded in ../../cmd/gc/reflect.c and
// ../reflect/type.go. Don't change this structure without also changing that code!
uintgo count; // # live cells == size of map. Must be first (used by len() builtin)
uint32 flags;
uint32 hash0; // hash seed
uint8 B; // log_2 of # of buckets (can hold up to LOAD * 2^B items)
uint8 keysize; // key size in bytes
uint8 valuesize; // value size in bytes
uint16 bucketsize; // bucket size in bytes
byte *buckets; // array of 2^B Buckets. may be nil if count==0.
byte *oldbuckets; // previous bucket array of half the size, non-nil only when growing
uintptr nevacuate; // progress counter for evacuation (buckets less than this have been evacuated)
};
// possible flags
enum
{
IndirectKey = 1, // storing pointers to keys
IndirectValue = 2, // storing pointers to values
Iterator = 4, // there may be an iterator using buckets
OldIterator = 8, // there may be an iterator using oldbuckets
};
// Macros for dereferencing indirect keys
#define IK(h, p) (((h)->flags & IndirectKey) != 0 ? *(byte**)(p) : (p))
#define IV(h, p) (((h)->flags & IndirectValue) != 0 ? *(byte**)(p) : (p))
// If you modify Hiter, also change cmd/gc/reflect.c to indicate
// the layout of this structure.
struct Hiter
{
uint8* key; // Must be in first position. Write nil to indicate iteration end (see cmd/gc/range.c).
uint8* value; // Must be in second position (see cmd/gc/range.c).
MapType *t;
Hmap *h;
byte *buckets; // bucket ptr at hash_iter initialization time
struct Bucket *bptr; // current bucket
uint8 offset; // intra-bucket offset to start from during iteration (should be big enough to hold BUCKETSIZE-1)
bool done;
// state of table at time iterator is initialized
uint8 B;
// iter state
uintptr bucket;
uintptr i;
intptr check_bucket;
};