The repo file format / razor_set data structure
-----------------------------------------------

The repo starts with a header, containing some number of sections,
terminated by a section with type 0:

	struct razor_set_header {
		uint32_t magic;
		uint32_t version;
		struct razor_set_section sections[0];
	};

	struct razor_set_section {
		uint32_t type;
		uint32_t offset;
		uint32_t size;
	};

razor_set_open() mmaps the repo file, and creates a struct razor_set:

	struct razor_set {
		struct array string_pool;
	 	struct array packages;
	 	struct array properties;
	 	struct array files;
		struct array package_pool;
	 	struct array property_pool;
	 	struct array file_pool;
		struct razor_set_header *header;
	};

by finding the sections with those IDs and creating "struct array"s
pointing to the right places in the mmapped data. (This is the only
processing needed when reading in the file; everything else is used
exactly as-is.)


The sections
------------

RAZOR_STRING_POOL

	Stores one copy of each string that appears in the repo. (At
	the moment, this is: package names, package versions, property
	names, property versions, and (basenames of) filenames.) The
	strings are arbitrarily-sized, 0-terminated, and not in any
	particular order (although the empty string always ends up
	being at offset 0).

RAZOR_PACKAGES

	Array of struct razor_package; one for each package in the
	set, sorted by name.

RAZOR_PROPERTIES

	Array of struct razor_property; one for each unique property
	in the set, sorted by type, then name, then relation type (eg,
	"<" or ">="), then version. (Properties with no version have
	relation type RAZOR_VERSION_EQUAL, and version "".)

RAZOR_FILES

	Array of struct razor_entry; one for each file owned by any
	package in the set. The current sort order (which is subject
	to change) is breadth-first, sorted by basename. So eg: /, /bin,
	/dev, /etc, /bin/false, /bin/true, /dev/null, /etc/passwd.

RAZOR_PACKAGE_POOL

	Array of struct list, with each list item containing the index
	of a struct razor_package in the packages section. See the
	discussion of lists below.

RAZOR_PROPERTY_POOL

	Array of struct list, with each list item containing the index
	of a struct razor_property in the properties section. See the
	discussion of lists below.

RAZOR_FILE_POOL

	Array of struct list, with each list item containing the index
	of a struct razor_entry in the files section. See the
	discussion of lists below.


Data types
----------
Note that the exact layout of bits involves some historical accidents.
(Particularly the fact that the "name" field in most structs loses its
high bits to a flags field.)

struct list_head
	uint list_ptr : 24;
	uint flags    : 8;

struct list
	uint data  : 24;
	uint flags : 8;

	Used to store lists of package, property, or file IDs. "struct
	list_head" stores the head of the list, which points to one or
	more "struct list"s in the appropriate "pool" section.
	("struct list" should probably be called "struct list_item".)

	"list_first(&head, &pool)" returns a "struct list *" pointing
	to the first element of the list (or NULL for an empty list),
	and "list_next(list)" will return successive elements, until
	NULL is returned. Each "list->data" contains the index of a
	package, property, or file in the corresponding section of the
	set.

	Peeking underneath the abstraction, a list_head's "flags" is
	0xff if the list is empty, 0x80 if it contains a single
	element, or 0x00 if it contains more than one element. In the
	single-element case, that element is actually stored in the
	list_head directly rather than being stored in a pool (and so
	list_first() just casts the list_head* to a list* and returns
	it). For multi-element lists, list_ptr is the index in the
	pool of the first element of this list; the list continues
	through successive elements of the pool until one with
	non-zero flags is reached, indicating the end of the list.

struct razor_package
	uint name    : 24;
	uint flags   : 8;
	uint version : 32;
	struct list_head properties;
	struct list_head files;

	name and version are indexes into string_pool. properties is a
	list of all of the package's properties, and files is a list
	of its files. flags is currently only used during razor_set
	merging, to keep track of which set a package came from.

struct razor_property
	uint name     : 24;
	uint flags    : 6;
	uint type     : 2;
	uint relation : 32;
	uint version  : 32;
	struct list_head packages;

	name and version are indexes into string_pool. type is an enum
	razor_property_type (eg, RAZOR_PROPERTY_REQUIRES), and
	relation is an enum razor_version_relation (eg,
	RAZOR_VERSION_GREATER_OR_EQUAL). packages is a list of the
	packages that have this property. flags is currently unused.

struct razor_entry
	uint name  : 24;
	uint flags : 8;
	uint start : 32;
	struct list_head packages;

	name is an index into string_pool, giving the basename of the
	file. start is either 0, or an index pointing to another
	razor_entry that is the first child of this entry (for a
	non-empty directory). (Entry 0 is always the root of the tree,
	so no entry could have entry 0 as a child.) flags is 0x80
	(RAZOR_ENTRY_LAST) if an entry is the last entry in its
	directory. Otherwise it is 0.

	Note that given a pointer to a struct_razor_entry (eg, from a
	package's "files" list), there is no way to reconstruct its
	full name without walking the entire files array up to that
	point. Because of this and other problems (fix_file_map()), it
	seems like razor_entry should be modified to include a pointer
	to its parent. (Storing full paths instead of just basenames
	would also fix this problem, but that would use much more
	memory.)