[strings] Add optimized utilities to deal with path extensions

strings.lua

@@ -8,6 +8,100 @@ local strings = {}
 
 -- Platform preferred path separator
 local SEP = package.config:sub(1,1)
+local DOT = string.byte('.')
+local SLASH = string.byte('/')
+local BACKSLASH = string.byte('\\')
+local COLON = string.byte(':')
+
+--- Find file extension within path.
+--
+-- @string path a file path
+-- @string[opt] sep path separator pattern, any combination of '/', '\\' or ':' to support various OSes, defaults to all
+-- @treturn number extension position within path if an extension is found, nil otherwise
+function strings.findpathext(path, sep)
+    sep = sep or '/\\:'
+    for i = 1,#sep do
+        local byt = sep:byte(i)
+        if byt ~= SLASH and byt ~= BACKSLASH and byt ~= COLON then
+            error(("Unsupported path separator pattern: %q"):format(sep))
+        end
+    end
+
+    local function issep(byt)
+        for i = 1,#sep do
+                if byt == sep:byte(i) then return true end
+        end
+    end
+
+    local pos = nil
+
+    for i = #path,2,-1 do
+        local byt = path:byte(i)
+
+        if byt == DOT and not issep(path:byte(i-1)) then
+            -- Update extension position
+            pos = i
+        elseif issep(byt) then
+            break
+        end
+    end
+
+    return pos
+end
+
+--- Set default file extension.
+--
+-- If path contains an extension, returns the path unaltered,
+-- otherwise set extension to the specified one.
+--
+-- @string path a file path
+-- @string ext default extension to be set
+-- @string[opt] sep path separator pattern, any combination of '/', '\\' or ':' to support various OSes, defaults to all
+-- @treturn string updated path, and separator position
+function strings.setdefpathext(path, ext, sep)
+    if ext:byte(1) ~= DOT then
+        error(("Bad extension %q: must be a string starting with '.'"):format(ext))
+    end
+
+    local pos = strings.findpathext(path, sep)
+    if not pos then
+        -- Append default extension
+        pos = #path
+        path = path..ext
+    end
+
+    return path, pos
+end
+
+--- Set file extension.
+--
+-- @string path a file path
+-- @string ext extension to be set (including '.')
+-- @string[opt] sep path separator pattern, any combination of '/', '\\' or ':' to support various OSes, defaults to all
+-- @treturn string updated path and separator position
+function strings.setpathext(path, ext, sep)
+    if ext:byte(1) ~= DOT then
+        error(("Bad extension %q: must be a string starting with '.'"):format(ext))
+    end
+
+    local pos = strings.findpathext(path, sep)
+    if pos then
+        -- Trim existing extension
+        path = path:sub(1, pos-1)
+    end
+
+    return path..ext, pos
+end
+
+--- Get file extension.
+--
+-- @string path a file path
+-- @string[opt] sep path separator pattern, any combination of '/', '\\' or ':' to support various OSes, defaults to all
+-- @treturn string file extension and position, if any is found, nil otherwise
+function strings.getpathext(path, sep)
+    local pos = strings.findpathext(path, sep)
+    if pos then return path:sub(pos), pos end
+end
 
 --- Remove redundant slashes and resolve dot and dot-dots in path.
 --