feat: add compressedSize and :compressionEfficiency()

Adds a field for getting the compressed size of a zip entry and a method
to get the compression efficiency as a percentage.

lib/init.luau

@@ -39,6 +39,7 @@ local DECOMPRESSION_ROUTINES: { [number]: { name: CompressionMethod, decompress:
 
 local EMPTY_PROPERTIES: ZipEntryProperties = table.freeze({
 	versionMadeBy = 0,
+	compressedSize = 0,
 	size = 0,
 	attributes = 0,
 	timestamp = 0,
@@ -79,6 +80,7 @@ type ZipEntryInner = {
 		os: MadeByOS,          -- Operating system used to create the ZIP
 	},
 
+	compressedSize: number,    -- Compressed size in bytes
 	size: number,              -- Uncompressed size in bytes
     offset: number,            -- Absolute position of local header in ZIP
     timestamp: number,         -- MS-DOS format timestamp
@@ -117,6 +119,7 @@ export type MadeByOS =
 export type CompressionMethod = "STORE" | "DEFLATE"
 export type ZipEntryProperties = {
 	versionMadeBy: number,
+	compressedSize: number,
 	size: number,
 	attributes: number,
 	timestamp: number,
@@ -135,6 +138,7 @@ function ZipEntry.new(offset: number, name: string, properties: ZipEntryProperti
 				software = string.format("%d.%d", versionMadeByVersion / 10, versionMadeByVersion % 10),
 				os = MADE_BY_OS_LOOKUP[versionMadeByOS] :: MadeByOS,
 			},
+			compressedSize = properties.compressedSize,
 			size = properties.size,
 			offset = offset,
 			timestamp = properties.timestamp,
@@ -179,6 +183,15 @@ function ZipEntry.sanitizePath(self: ZipEntry): string
 	return path.sanitize(pathStr)
 end
 
+function ZipEntry.compressionEfficiency(self: ZipEntry): number?
+	if self.size == 0 or self.compressedSize == 0 then 
+		return nil
+	end
+
+	local ratio = 1 - self.compressedSize / self.size
+	return math.round(ratio * 100)
+end
+
 -- TODO: More methods for `ZipEntry`, handle octals and unix perms
 
 local ZipReader = {}
@@ -260,6 +273,7 @@ function ZipReader.parseCentralDirectory(self: ZipReader): ()
 		-- 10      2      Compression method (8 = DEFLATE)
 		-- 12      4      Last mod time/date
 		-- 16      4      CRC-32
+		-- 20      4      Compressed size
 		-- 24      4      Uncompressed size
 		-- 28      2      File name length (n)
 		-- 30      2      Extra field length (m)
@@ -276,6 +290,7 @@ function ZipReader.parseCentralDirectory(self: ZipReader): ()
 		local timestamp = buffer.readu32(self.data, pos + 12)
 		local compressionMethod = buffer.readu16(self.data, pos + 10)
 		local crc = buffer.readu32(self.data, pos + 16)
+		local compressedSize = buffer.readu32(self.data, pos + 20)
 		local size = buffer.readu32(self.data, pos + 24)
 		local nameLength = buffer.readu16(self.data, pos + 28)
 		local extraLength = buffer.readu16(self.data, pos + 30)
@@ -289,6 +304,7 @@ function ZipReader.parseCentralDirectory(self: ZipReader): ()
 			self.entries,
 			ZipEntry.new(offset, name, {
 				versionMadeBy = versionMadeBy,
+				compressedSize = compressedSize,
 				size = size,
 				crc = crc,
 				method = DECOMPRESSION_ROUTINES[compressionMethod].name :: CompressionMethod,
@@ -341,6 +357,7 @@ function ZipReader.buildDirectoryTree(self: ZipReader): ()
 						-- parent directories in the ZIP
 						local dir = ZipEntry.new(0, path .. "/", {
 							versionMadeBy = 0,
+							compressedSize = 0,
 							size = 0,
 							crc = 0,
 							compressionMethod = "STORED",

tests/metadata.luau

@@ -114,8 +114,7 @@ return function(test: typeof(frktest.test))
 							and not string.match(line, "files?$")
 							and #line > 0
 						then
-							-- TODO: Expose information about size, and compression ratio in API
-							local length, method, _size, _cmpr, expectedDate, expectedTime, crc32, name = string.match(
+							local length, method, size, cmpr, expectedDate, expectedTime, crc32, name = string.match(
 								line,
 								"^%s*(%d+)%s+(%S+)%s+(%d+)%s+([+-]?%d*%%?)%s+(%d%d%d%d%-%d%d%-%d%d)%s+(%d%d:%d%d)%s+(%x+)%s+(.+)$"
 							)
@@ -134,11 +133,18 @@ return function(test: typeof(frktest.test))
 								check.equal(OS_NAME_TRANSFORMATIONS[assert(versionMadeByOS)], entry.versionMadeBy.os)
 							end
 
+							local gotCmpr = entry:compressionEfficiency()
 							local gotDateTime = DateTime.fromLocalTime(
 								timestampToValues(entry.timestamp) :: DateTime.DateTimeValueArguments
 							)
 
 							check.equal(tonumber(length), entry.size)
+							check.equal(tonumber(size), entry.compressedSize)
+
+							if gotCmpr ~= nil then
+								check.equal(cmpr, gotCmpr .. "%")
+							end
+							
 							check.equal(METHOD_NAME_TRANSFORMATIONS[method :: string], entry.method)
 							check.is_true(
 								dateFuzzyEq(gotDateTime:formatLocalTime("%Y-%m-%d"), expectedDate :: string, 1)