[Sync Iteration] arm64-assembly/collatz-conjecture/1

solutions/arm64-assembly/collatz-conjecture/1/collatz_conjecture.s

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+// collatz_conjecture.s
+
+// Implements Collatz conjecture
+
+.equ ERROR_INVALID_NUMBER, -1
+.equ INVALID, 0
+.equ COUNTER_INIT, 0
+.equ BASE_CASE, 1
+.equ DIVISION_BASE, 1
+.equ INCREMENT, 1
+.equ IS_EVEN, 1
+.equ FACTOR, 3
+
+.global steps
+steps:
+    // x0 has the number (16)
+    // x1 will be the step counter
+    cmp x0, #INVALID                // is it 0?
+    ble error                       // exits
+
+    mov x1, #COUNTER_INIT           // counter = 0
+
+loop:
+    cmp x0, #BASE_CASE              // compares x0 to 1
+    beq done                        // when equal, exits loop
+
+    tst x0, #IS_EVEN                // is it even?
+    beq even                        // if so, go to division
+
+    mov x2, #FACTOR                 // stores the factor 3
+    mul x0, x0, x2                  // multiplies
+    add x0, x0, #INCREMENT          // adds 1
+
+    add x1, x1, #INCREMENT          // counter++
+    b loop                          // returns to loop    
+
+even:    
+    asr x0, x0, #DIVISION_BASE      // divides by 2
+
+    add x1, x1, #INCREMENT          // counter++
+    b loop                          // returns to loop
+
+done:
+    mov x0, x1                      // returns counter
+    ret                             // farewell
+
+error:
+    mov x0, #ERROR_INVALID_NUMBER   // error message
+    ret                             // bye
+