% UTF-8 string routines

# Introduction

Here we should give a short overview of Unicode/UCS and in particular UTF-8
encoding.

Explain about code points and relationship to "characters".

Explain about half-open intervals.

Always be careful whether a function takes byte offsets or code-point indices.
In general, all position parameters are in byte offsets, not code point indices.
This may be surprising, but it is because the functions are designed to be
highly performant and to also work with arbitrary byte buffers. Therefore UTF8
decoding is not done by default.

For actual text processing, where you want to specify positions with code point
indices, you should use [al_ustr_offset] to find the byte position of
a code point.

# UTF-8 string types

## API: ALLEGRO_USTR


## API: ALLEGRO_USTR_INFO


# Creating and destroying strings

## API: al_ustr_new

Create a new string containing a copy of the C-style string `s`.
The string must eventually be freed with [al_ustr_free].


## API: al_ustr_new_from_buffer

Create a new string containing a copy of the buffer pointed to by `s` of the
given size.  The string must eventually be freed with [al_ustr_free].


## API: al_ustr_newf

Create a new string using a printf-style format string.

Notes:

That "%s" specifier takes C string arguments, not ALLEGRO_USTRs.  Therefore to
pass an ALLEGRO_USTR as a parameter you must use [al_cstr], and it must be NUL
terminated.  If the string contains an embedded NUL byte everything from that
byte onwards will be ignored.

The "%c" specifier outputs a single byte, not the UTF-8 encoding of a code
point.  Therefore it's only usable for ASCII characters (value <= 127) or if
you really mean to output byte values from 128--255.  To insert the UTF-8
encoding of a code point, encode it into a memory buffer using
[al_utf8_encode] then use the "%s" specifier.  Remember to NUL terminate the
buffer.


## API: al_ustr_free

Free a previously allocated string.


## API: al_cstr

Get a `char *` pointer to the data in a string.  This pointer will only be
valid while the underlying string is not modified and not destroyed.
The pointer may be passed to functions expecting C-style strings,
with the following caveats:

* ALLEGRO_USTRs are allowed to contain embedded NUL ('\0') bytes.
  That means `al_ustr_size(u)` and `strlen(al_cstr(u))` may not agree.

* An ALLEGRO_USTR may be created in such a way that it is not NUL terminated.
  A string which is dynamically allocated will always be NUL terminated,
  but a string which references the middle of another string or region
  of memory will *not* be NUL terminated.

* If the ALLEGRO_USTR references another string, the returned c-string will
  point into the referenced string, the length of the string will be ignored.

## API: al_ustr_to_buffer

Write the contents of the string into a pre-allocated buffer of the given size
in bytes. The result will always be 0-terminated.

## API: al_cstr_dup

Create a NUL  ('\0') terminated copy of the string.  Any embedded NUL
bytes will still be presented in the returned string.
The new string must eventually be freed with free().
If an error occurs NULL is returned.

[after we introduce al_free it should be freed with al_free]


## API: al_ustr_dup

Return a duplicate copy of a string.
The new string will need to be freed with [al_ustr_free].


## API: al_ustr_dup_substr

Return a new copy of a string, containing its contents in the byte interval
\[start_pos, end_pos).  The new string will be NUL terminated and will
need to be freed with [al_ustr_free].

If you need a range of code-points instead of bytes, use [al_ustr_offset]
to find the byte offsets.


# Predefined strings

## API: al_ustr_empty_string

Return a pointer to a static empty string.  The string is read only.


# Creating strings by referencing other data

## API: al_ref_cstr

Create a string that references the storage of a C-style string.  The
information about the string (e.g. its size) is stored in the structure
pointed to by the `info` parameter.  The string will not have any other
storage allocated of its own, so if you allocate the `info` structure on the
stack then no explicit "free" operation is required.

The string is valid until the underlying C string disappears.

Example:

    ALLEGRO_USTR_INFO info;
    ALLEGRO_USTR us = al_ref_cstr(&info, "my string");


## API: al_ref_buffer

Like [al_ref_cstr] but the size of the string data is passed in as a
parameter.  Hence you can use it to reference only part of a string or an
arbitrary region of memory.

The string is valid while the underlying C string is valid.


## API: al_ref_ustr

Create a read-only string that references the storage of another string.
The information about the string (e.g. its size) is stored in the structure
pointed to by the `info` parameter.  The string will not have any other
storage allocated of its own, so if you allocate the `info` structure on the
stack then no explicit "free" operation is required.

The referenced interval is \[start_pos, end_pos).

The string is valid until the underlying string is modified or destroyed.

If you need a range of code-points instead of bytes, use [al_ustr_offset]
to find the byte offsets.


# Sizes and offsets

## API: al_ustr_size

Return the size of the string in bytes.  This is equal to the number of code
points in the string if the string is empty or contains only 7-bit ASCII
characters.

## API: al_ustr_length

Return the number of code points in the string.

## API: al_ustr_offset

Return the offset (in bytes from the start of the string) of the code point at
the specified index in the string. A zero index parameter will return the first
character of the string. If index is negative, it counts backward from the end
of the string, so an index of -1 will return an offset to the last code point.

If the index is past the end of the string, returns the offset of the end of
the string.

## API: al_ustr_next

Find the byte offset of the next code point in string, beginning at `*pos`.
`*pos` does not have to be at the beginning of a code point.
Returns true on success, then value pointed to by `pos` will be updated to the
found offset.  Otherwise returns false if `*pos` was already at the end of
the string, then `*pos` is unmodified.

This function just looks for an appropriate byte; it doesn't check if found
offset is the beginning of a valid code point.  If you are working with
possibly invalid UTF-8 strings then it could skip over some invalid bytes.

## API: al_ustr_prev

Find the byte offset of the previous code point in string, before `*pos`.
`*pos` does not have to be at the beginning of a code point.
Returns true on success, then value pointed to by `pos` will be updated to the
found offset.  Otherwise returns false if `*pos` was already at the end of
the string, then `*pos` is unmodified.

This function just looks for an appropriate byte; it doesn't check if found
offset is the beginning of a valid code point.  If you are working with
possibly invalid UTF-8 strings then it could skip over some invalid bytes.


# Getting code points

## API: al_ustr_get

Return the code point in `us` beginning at `pos`.

On success returns the code point value.
If `pos` was out of bounds (e.g. past the end of the string), return -1.
On an error, such as an invalid byte sequence, return -2.

## API: al_ustr_get_next

Find the code point in `us` beginning at `*pos`, then advance to the next
code point.

On success return the code point value.
If `pos` was out of bounds (e.g. past the end of the string), return -1.
On an error, such as an invalid byte sequence, return -2.
As with [al_ustr_next], invalid byte sequences may be skipped while advancing.

## API: al_ustr_prev_get

Find the beginning of a code point before `*pos`, then return it.
Note this performs a *pre-increment*.

On success returns the code point value.
If `pos` was out of bounds (e.g. past the end of the string), return -1.
On an error, such as an invalid byte sequence, return -2.
As with [al_ustr_prev], invalid byte sequences may be skipped while advancing.


# Inserting into strings

## API: al_ustr_insert

Insert `us2` into `us1` beginning at `pos`.  `pos` cannot be less than 0.  If
`pos` is past the end of `us1` then the space between the end of the string
and `pos` will be padded with NUL ('\0') bytes. `pos` is specified in bytes.

Use [al_ustr_offset] to find the byte offset for a code-points offset

Returns true on success, false on error.

## API: al_ustr_insert_cstr

Like [al_ustr_insert] but inserts a C-style string.

## API: al_ustr_insert_chr

Insert a code point into `us` beginning at byte offset `pos`.  `pos` cannot be
less than 0. If `pos` is past the end of `us` then the space between the end of
the string and `pos` will be padded with NUL ('\0') bytes.

Returns the number of bytes inserted, or 0 on error.


# Appending to strings

## API: al_ustr_append

Append `us2` to the end of `us1`.

Returns true on success, false on error.

## API: al_ustr_append_cstr

Append C-style string `s` to the end of `us`.

Returns true on success, false on error.

## API: al_ustr_append_chr

Append a code point to the end of `us`.

Returns the number of bytes added, or 0 on error.

## API: al_ustr_appendf

This function appends formatted output to the string `us`.  `fmt` is a
printf-style format string.
See [al_ustr_newf] about the "%s" and "%c" specifiers.

Returns true on success, false on error.

## API: al_ustr_vappendf

Like [al_ustr_appendf] but you pass the variable argument list directly,
instead of the arguments themselves.
See [al_ustr_newf] about the "%s" and "%c" specifiers.

Returns true on success, false on error.


# Removing parts of strings

## API: al_ustr_remove_chr

Remove the code point beginning at byte offset `pos`.  Returns true on success.
If `pos` is out of range or `pos` is not the beginning of a valid code point,
returns false leaving the string unmodified.

Use [al_ustr_offset] to find the byte offset for a code-points offset.

## API: al_ustr_remove_range

Remove the interval \[start_pos, end_pos) (in bytes) from a string.
`start_pos` and `end_pos` may both be past the end of the string
but cannot be less than 0 (the start of the string).

Returns true on success, false on error.

## API: al_ustr_truncate

Truncate a portion of a string at byte offset `start_pos` onwards.
`start_pos` can be past the end of the string (has no effect)
but cannot be less than 0.

Returns true on success, false on error.

## API: al_ustr_ltrim_ws

Remove leading whitespace characters from a string, as defined by the C
function `isspace()`.

Returns true on success, or false if the function was passed an empty string.

## API: al_ustr_rtrim_ws

Remove trailing ("right") whitespace characters from a string, as defined by
the C function `isspace()`.

Returns true on success, or false if the function was passed an empty string.

## API: al_ustr_trim_ws

Remove both leading and trailing whitespace characters from a string.

Returns true on success, or false if the function was passed an empty string.


# Assigning one string to another

## API: al_ustr_assign

Overwrite the string `us1` with another string `us2`.
Returns true on success, false on error.

## API: al_ustr_assign_substr

Overwrite the string `us1` with the contents of `us2` in the byte interval
\[start_pos, end_pos).  The end points will be clamed to the bounds of `us2`.

Usually you will first have to use [al_ustr_offset] to find the byte offsets.

Returns true on success, false on error.

## API: al_ustr_assign_cstr

Overwrite the string `us` with the contents of the C-style string `s`.
Returns true on success, false on error.


# Replacing parts of string

## API: al_ustr_set_chr

Replace the code point beginning at byte offset `pos` with `c`.
`pos` cannot be less than 0.  If `pos` is past the end of `us1` then the space
between the end of the string and `pos` will be padded with NUL ('\0') bytes.
If `pos` is not the start of a valid code point, that is an error and
the string will be unmodified.

On success, returns the number of bytes written, i.e. the offset to the
following code point.  On error, returns 0.

## API: al_ustr_replace_range

Replace the part of `us1` in the byte interval \[start_pos, end_pos) with the
contents of `us2`.  `start_pos` cannot be less than 0.  If `start_pos` is past
the end of `us1` then the space between the end of the string and `start_pos`
will be padded with NUL ('\0') bytes.

Use [al_ustr_offset] to find the byte offsets.

Returns true on success, false on error.


# Searching

## API: al_ustr_find_chr

Search for the encoding of code point `c` in `us` from byte offset `start_pos`
(inclusive).

Returns the position where it is found or -1 if it is not found.

## API: al_ustr_rfind_chr

Search for the encoding of code point `c` in `us` backwards from byte offset
`end_pos` (exclusive).
Returns the position where it is found or -1 if it is not found.

## API: al_ustr_find_set

This function finds the first code point in `us`, beginning from byte offset
`start_pos`, that matches any code point in `accept`.  Returns the position if
a code point was found.  Otherwise returns -1.

## API: al_ustr_find_set_cstr

Like [al_ustr_find_set] but takes a C-style string for `accept`.

## API: al_ustr_find_cset

This function finds the first code point in `us`, beginning from byte offset
`start_pos`, that does *not* match any code point in `reject`.  In other words
it finds a code point in the complementary set of `reject`.
Returns the byte position of that code point, if any.  Otherwise returns -1.

## API: al_ustr_find_cset_cstr

Like [al_ustr_find_cset] but takes a C-style string for `reject`.

## API: al_ustr_find_str

Find the first occurrence of string `needle` in `haystack`, beginning from byte
offset `pos` (inclusive).  Return the byte offset of the occurrence if it is
found, otherwise return -1.

## API: al_ustr_find_cstr

Like [al_ustr_find_str] but takes a C-style string for `needle`.

## API: al_ustr_rfind_str

Find the last occurrence of string `needle` in `haystack` before byte offset
`end_pos` (exclusive).  Return the byte offset of the occurrence if it is found,
otherwise return -1.

## API: al_ustr_rfind_cstr

Like [al_ustr_rfind_str] but takes a C-style string for `needle`.

## API: al_ustr_find_replace

Replace all occurrences of `find` in `us` with `replace`, beginning at byte
offset `start_pos`.  The `find` string must be non-empty.
Returns true on success, false on error.

## API: al_ustr_find_replace_cstr

Like [al_ustr_find_replace] but takes C-style strings for `find` and `replace`.


# Comparing

## API: al_ustr_equal

Return true iff the two strings are equal.  This function is more efficient
than [al_ustr_compare] so is preferable if ordering is not important.

## API: al_ustr_compare

This function compares `us1` and `us2` by code point values.
Returns zero if the strings are equal, a positive number if `us1` comes after
`us2`, else a negative number.

This does *not* take into account locale-specific sorting rules.
For that you will need to use another library.

## API: al_ustr_ncompare

Like [al_ustr_compare] but only compares up to the first `n` code points
of both strings.

Returns zero if the strings are equal, a positive number if `us1` comes after
`us2`, else a negative number.

## API: al_ustr_has_prefix

Returns true iff `us1` begins with `us2`.

## API: al_ustr_has_prefix_cstr

Returns true iff `us1` begins with `s2`.

## API: al_ustr_has_suffix

Returns true iff `us1` ends with `us2`.

## API: al_ustr_has_suffix_cstr

Returns true iff `us1` ends with `s2`.


# UTF-16 conversion

## API: al_ustr_new_from_utf16

Create a new string containing a copy of the 0-terminated string `s`
which must be encoded as UTF-16.
The string must eventually be freed with [al_ustr_free].

## API: al_ustr_size_utf16

Returns the number of bytes required to encode the string in UTF-16
(including the terminating 0). Usually called before
[al_ustr_encode_utf16] to determine the size of the buffer to allocate.

## API: al_ustr_encode_utf16

Encode the string into the given buffer, in UTF-16. Returns the number
of bytes written. There are never more than `n` bytes written. The
minimum size to encode the complete string can be queried with
[al_ustr_size_utf16]. If the `n` parameter is smaller than that, the
string will be truncated but still always 0 terminated.


# Low-level UTF-8 routines

## API: al_utf8_width

Returns the number of bytes that would be occupied by the specified code point
when encoded in UTF-8.  This is between 1 and 4 bytes for legal code point
values.  Otherwise returns 0.

## API: al_utf8_encode

Encode the specified code point to UTF-8 into the buffer `s`.  The buffer
must have enough space to hold the encoding, which takes between 1 and 4
bytes.  This routine will refuse to encode code points above 0x10FFFF.

Returns the number of bytes written, which is the same as that returned
by [al_utf8_width].


# Low-level UTF-16 routines

## API: al_utf16_width

Returns the number of bytes that would be occupied by the specified code
point when encoded in UTF-16. This is either 2 or 4 bytes for legal code
point values. Otherwise returns 0.

## API: al_utf16_encode

Encode the specified code point to UTF-8 into the buffer `s`. The buffer
must have enough space to hold the encoding, which takes either 2 or 4
bytes. This routine will refuse to encode code points above 0x10FFFF.

Returns the number of bytes written, which is the same as that returned
by [al_utf16_width].


