Add booleans are wasted memory post

content/posts/2020-11-17-booleans-are-wasted-memory.md

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+---
+title: Booleans are wasted memory
+date: "2020-11-06"
+---
+
+A boolean is either `true` or `false`. That translates to 1 or 0. If you think that one bit is enough to store this information, you'd be wrong.
+
+In order to keep the binary layout of a program simple and convenient, most languages store information in either 4 bit or 8 bit blocks. If you allocate a `bool` in C, you will get [an integer constant with a value of either 1 or 0](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/stdbool.h.html), taking up 4 bits.
+If you allocate a bool in Rust or (most) other languages that are based on LLVM, [it will take up 1 `i1`, or 8 bit of memory](https://llvm.org/docs/LangRef.html#simple-constants).
+
+## There's a better way
+
+If you find yourself having to store multiple boolean states somewhere, you might simply declare those booleans and call it a day:
+
+```c
+#include <stdbool.h>
+#include <stdio.h>
+int main()
+{
+    bool can_read = true;
+    bool can_write = false;
+    bool can_execute = true;
+
+    if (can_read)
+        printf("read bit set\n");
+    if (can_write)
+        printf("write bit set\n");
+    if (can_execute)
+        printf("execute bit set\n");
+
+    // Output:
+    // read bit set
+    // write bit set
+}
+```
+
+An alternative approach to store boolean values is to share a "chunk" of bits with other values. This is usually done using bitwise operations:
+
+```c
+#include <stdbool.h>
+#include <stdio.h>
+
+// Define permissions
+#define PERM_NONE       0b000
+#define PERM_READ       0b001
+#define PERM_WRITE      0b010
+#define PERM_EXECUTE    0b100
+
+#define PERM_ALL        PERM_READ | PERM_WRITE | PERM_EXECUTE
+
+int main()
+{
+    // Set permissions
+    int permissions = PERM_READ | PERM_WRITE;
+
+    if (permissions & PERM_READ)
+        printf("write bit set\n");
+    if (permissions & PERM_WRITE)
+        printf("read bit set\n");
+    if (permissions & PERM_EXECUTE)
+        printf("execute bit set\n");
+
+    // Output:
+    // read bit set
+    // write bit set
+}
+```
+
+This example still wastes 1 bit since we only use 3 out of 4 possible bits of the integer type, but I'm sure you get the point. Whenever you find yourself needing multiple boolean values, think twice if you can use this pattern.
+
+Microcontrollers have a very constrainted environment, therefore bitwise operations are essential in those scenarios. 7 wasted bytes are a lot if there are only 4 kb of total memory available. For larger systems we often forget about these constraints, until they add up.
+
+## My Plea
+* Be mindful about the software you create.
+* Appreciate the resources at your disposal.