docs(usage) fix m_cost description and provide better examples

CHANGELOG.md

@@ -12,7 +12,7 @@
 
 ### Added
 
-- :stars: Support for Argon2_id encoding variant.
+- :stars: Support for Argon2id encoding variant.
   [#24](https://github.com/thibaultcha/lua-argon2/pull/24)
 - We now automatically compute the length of the retrieved encoded hash from
   `encrypt()`. [#21](https://github.com/thibaultcha/lua-argon2/pull/21)
@@ -78,7 +78,7 @@
 
 Initial release with support for Argon2
 [20151206](https://github.com/P-H-C/phc-winner-argon2/releases/tag/20151206).
-Implement a C bridge for Argon2_i and Argon2_d encoding and a
+Implement a C bridge for Argon2i and Argon2d encoding and a
 Lua module for input validation.
 
 [unreleased]: https://github.com/thibaultcha/lua-argon2/compare/3.0.0...master

README.md

@@ -99,10 +99,10 @@ local encoded = assert(argon2.encrypt("password", "somesalt", {
 -- Hashing options
 local encoded = assert(argon2.encrypt("password", "somesalt", {
   t_cost = 4,
-  m_cost = 16,
+  m_cost = math.pow(2, 16), -- 65536 KiB
   parallelism = 2
 }))
--- encoded is "$argon2i$v=19$m=16,t=4,p=2$c29tZXNhbHQ$zeUdR8y+Dqo4B5gDTYJzYVlJjVeg2rY5Le1/pp/wdVQ"
+-- encoded is "$argon2i$v=19$m=65536,t=4,p=2$c29tZXNhbHQ$n6x5DKNWV8BOeKemQJRk7BU3hcaCVomtn9TCyEA0inM"
 
 -- Changing the default options (those arguments are the current defaults)
 argon2.t_cost(3)

src/argon2.c

@@ -48,8 +48,10 @@ Default values of this module can be overriden with `m_cost()`, `t_cost()`,
     argon2.encrypt("password", "salt", { t_cost = 4 })
 Can be set to a new default in lua-argon2 (C binding only) by calling:
     argon2.t_cost(4)
-@field m_cost Sets memory usage to 2^N KiB (`number`, default: `12`).
-    argon2.encrypt("password", "salt", { m_cost = 16 })
+@field m_cost Sets memory usage as KiB (`number`, default: `12`).
+    argon2.encrypt("password", "salt", {
+      m_cost = math.pow(2, 16) -- 2^16 aka 65536 KiB
+    })
 Can be set to a new default in lua-argon2 (C binding only) by calling:
     argon2.m_cost(16)
 @field parallelism Number of threads and compute lanes (`number`, default: `1`).
@@ -234,9 +236,10 @@ if err then
   error("could not encrypt: " .. err)
 end
 
--- with options
+-- with options and variant
 local hash, err = argon2.encrypt("password", "somesalt", {
   t_cost = 4,
+  m_cost = math.pow(2, 16), -- 65536 KiB
   variant = argon2.variants.argon2_d
 })
 */