# Archive Map
An id-keyed container for the {ref}`TArchive` records of one index, laid out the same way
{ref}`TFileMap` is.

Config archives can also be addressed by their {ref}`EConfigArchive` name.


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## TArchiveMap._Init
```pascal
procedure TArchiveMap._Init(expectedCount: Integer);
```
Empties the map and sizes its hash table for roughly `expectedCount` entries.

The table is deliberately over-allocated - see {ref}`MapHashSize` - so that probing stays
short; {ref}`TArchiveMap._Grow` doubles it again if it ever fills past 60%.


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## TArchiveMap._FindSlot
```pascal
function TArchiveMap._FindSlot(key: Integer): Integer;
```
The hash slot belonging to `key`: either the one already holding it, or the first free one after
where it hashes to. Collisions walk forward one slot at a time and wrap around.


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## TArchiveMap._Grow
```pascal
procedure TArchiveMap._Grow();
```
Doubles the hash table and reinserts every key into it. Entry order is untouched - only the
slots that point at the entries are rebuilt.


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## ArchiveMap.Build
```pascal
procedure TArchiveMap.Build(const keys: TIntegerArray; count: Integer);
```
Fills the map in one pass with an empty {ref}`TArchive` per key, sizing the key, value and hash
arrays exactly once instead of regrowing them on every insert.

Only the first `count` entries of `keys` are used, so callers filling many maps in a row can
reuse one buffer. They must be in ascending order, which is how reference tables store them.


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## TArchiveMap.Exists
```pascal
function TArchiveMap.Exists(key: Integer): Boolean;
```
Whether the map holds an entry with this id.


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## TArchiveMap.IndexOf
```pascal
function TArchiveMap.IndexOf(key: Integer): Integer;
```
The position of this id in `Values`, or `-1` if it is not in the map.


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## TArchiveMap.Value
```pascal
property TArchiveMap.Value(key: Integer; const val: TArchive);
property TArchiveMap.Value(key: Integer): TArchive;
property TArchiveMap.Value(key: EConfigArchive; const val: TArchive);
property TArchiveMap.Value(key: EConfigArchive): TArchive;
```
Reads or writes the archive stored under `key`, which may be an id or, for the config index, an
{ref}`EConfigArchive` name. Reading an id the map does not hold raises.

Writing keeps `Values` sorted by id, with the same trade-off {ref}`TFileMap.Value` describes.


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## TArchiveMap.Clear
```pascal
procedure TArchiveMap.Clear();
```
Drops every entry and the hash table with them, leaving the map as if it had never been used.