add day9

day9/checksum.go

@@ -0,0 +1,93 @@
+package main
+
+import (
+    "os"
+    "fmt"
+    "strconv"
+    "strings"
+)
+
+
+// general util
+func check(e error) {
+    if e != nil { panic(e) }
+}
+
+func pop(arr []int, index *int) int {
+    /* get last element that is not a -1 and replacing it with -1 int the source slice */
+    for i := len(arr) - 1; i >= 0; i-- {
+        if arr[i] != -1 {
+            (*index) = i
+            elem := arr[i]
+            arr[i] = -1
+            return elem
+        }
+    }
+    return -1
+}
+
+func insert(arr []int, n int) int {
+    /* insert element at the first -1, return the index of this -1 */
+    for i, elem := range arr {
+        if elem == -1 {
+            arr[i] = n
+            return i
+        }
+    }
+    return -1
+}
+
+// main loop
+func main() {
+
+    // read in file
+    dat, err := os.ReadFile("data.txt")
+    check(err)
+    dat_str := strings.TrimSpace(string(dat))
+    
+    // contruct input as int arr 
+    input := make([]int, 0, len(dat_str))
+    mem_size := 0
+    for _, char := range []rune(dat_str) {
+        num, err := strconv.Atoi(string(char))
+        check(err)
+        mem_size += num
+        input = append(input, num)
+    }
+
+    // convert to actual memory representation
+    mem := make([]int, 0, mem_size)
+    is_file := true
+    file_id := 0
+    for _, elem := range input {
+        if is_file {
+            for range elem {
+                mem = append(mem, file_id)
+            }
+            file_id++
+        } else {
+            for range elem {
+                mem = append(mem, -1)
+            }
+        }
+        is_file = !is_file
+    }
+
+    // reorder
+    idx_pop    := -1
+    idx_insert := -2
+
+    for idx_pop != idx_insert {
+        elem := pop(mem, &idx_pop)
+        idx_insert = insert(mem, elem)
+    }
+
+    // calc checksum
+    checksum := 0
+    for i, elem := range mem {
+        if elem == -1 { break }
+        checksum += i * elem
+    }
+
+    fmt.Println(checksum)
+}