examples: add a tiny brainvuck interpreter (#16213)

examples/brainvuck.v

@@ -0,0 +1,66 @@
+import os
+
+fn main() {
+	if os.args.len < 2 {
+		eprintln('you need to supply a brainfuck program as a string argument')
+		exit(1) // exit with non-zero exit code if there is no program to run
+	}
+	program := os.args[1] // our program is fed in as a string
+
+	mut memory := []u8{len: 256} // we have 256 bytes of memory
+	mut address := u8(0) // as well as an 8-bit address register
+
+	mut stack := []int{} // our stack does not need a maximum length
+
+	mut program_counter := 0 // program counter
+
+	// interpreter starts here
+	for program_counter < program.len {
+		// we look at what the current character our program counter is seeing
+		match program[program_counter] {
+			`>` {
+				address++ // increment the address
+			}
+			`<` {
+				address-- // decrement the address
+			}
+			`+` {
+				memory[address]++ // increment the value at the address
+			}
+			`-` {
+				memory[address]-- // decrement the value at the address
+			}
+			`.` {
+				print(memory[address].ascii_str()) // print the value at the address
+			}
+			`,` {
+				input := os.input_opt('') or { '' } // read value and account for errors
+				memory[address] = input[0] // this is so we can ignore newlines
+				// because strings are 0-terminated, it also gives us a default value for free!
+			}
+			`[` {
+				stack << program_counter // add loop start address to the call stack
+			}
+			`]` {
+				if memory[address] != 0 {
+					// set the program counter to the last loop start
+					// so it jumps back and loops again
+					program_counter = stack[stack.len - 1]
+				} else {
+					// otherwise remove the address from the stack and continue
+					stack.pop()
+				}
+			}
+			else {
+				// the interpreter should ignore characters that are not part of the language
+			}
+		}
+		// increment the program counter to go to the next instruction
+		program_counter++
+		// back to line 20!
+	}
+
+	// print the state of the interpreter at the end
+	println('Address: $address')
+	println('Memory: $memory')
+}