advent-of-code/2024/day-9/solution-2.sh

#!/usr/bin/env bash

FILE=$1
# Sparse sequence
read -r -a SPARSE_ARRAY <<< "$(
  INDEX=0
  FILE_ID=0
  while read -r CHAR
  do
    if [[ $(( INDEX % 2 )) -eq 0 ]]
    then
      for (( i=1; i<=CHAR; i++ )) do printf "%s " "$FILE_ID"
      done
      (( FILE_ID++ ))
    else
      for (( i=1; i<=CHAR; i++ ))
      do
        printf "%s " '-1'
      done
    fi
    (( INDEX++ ))
  done <<< "$( sed -E 's/(.)/\1\n/g' "$FILE" | grep -v '^$' )"
)"
printf "%s" "${SPARSE_ARRAY[@]}" | fold -w 100
printf "\n"
printf "Sparse array len: %s\n" "${#SPARSE_ARRAY[@]}"
SHADOW_ARRAY=( "${SPARSE_ARRAY[@]}" )

# Start by working backwards
j=$(( ${#SPARSE_ARRAY[@]} - 1 )) # Moveable index since file blocks jump
k=0 # SHADOW_ARRAY cursor
LAST_SKIP=9999
while [[ $j -ge 0 ]]
do
  # Find a file
  if [[ ${SPARSE_ARRAY[j]} -eq -1 ]] 
  then
    # Not a file
    (( j-- ))
    continue
  fi

  # Get file size
  # j moved to index before file head
  FILE_SIZE=0
  ID=${SPARSE_ARRAY[j]}
  while [[ ${SPARSE_ARRAY[j]} == "$ID" ]]
  do
    (( FILE_SIZE++ ))
    (( j-- ))
  done
  #printf "File ID: %s, File size: %s\n" "$ID" "$FILE_SIZE" 2>&1

  # Since search space is constantly decreasing
  # Contiguous space can only keep getting smaller 
  if [[ $FILE_SIZE -ge $LAST_SKIP ]]
  then
    printf "File %s size %s skipped. Last skip: %s\n" "$ID" "$FILE_SIZE" "$LAST_SKIP"
    continue
  fi

  # Search for available contiguous space
  # Search from start until the file itself
  k=0
  while [[ $k -le $j ]]  && [[ $k -le ${#SHADOW_ARRAY[@]} ]]
  do
    CONT_SPACE=0

    # Skip ahead until first match of space
    while [[ ${SHADOW_ARRAY[k]} -ne -1 ]] && [[ $k -le ${#SHADOW_ARRAY[@]} ]]
    do
      (( k++ ))
    done

    # Space found, now get contiguous length
    while [[ ${SHADOW_ARRAY[k]} -eq -1 ]] && [[ $k -le ${#SHADOW_ARRAY[@]} ]]
    do
      (( CONT_SPACE++ ))
      (( k++ ))
    done
    
    # Space length is ok, can proceed
    if [[ $FILE_SIZE -le $CONT_SPACE ]] ;then break; fi
  done

  # Space length ok, can proceed
  if [[ $FILE_SIZE -le $CONT_SPACE ]]
  then
    # Move the file to head of contiguous space
    (( k-= CONT_SPACE ))
    while [[ $FILE_SIZE -gt 0 ]] && \
          [[ ${SHADOW_ARRAY[k]} -eq -1 ]] && \
          [[ ${SHADOW_ARRAY[j+FILE_SIZE]} -ne -1 ]]
    do
      SHADOW_ARRAY[k]=$ID
      SHADOW_ARRAY[j+FILE_SIZE]=-1
      (( k++ ))
      (( FILE_SIZE-- ))
    done
  else
    LAST_SKIP=$FILE_SIZE
    printf "File %s skipped. File size: %s\n" "$ID" "$FILE_SIZE"
    continue
  fi
done
printf "%s " "${SHADOW_ARRAY[@]}" | fold -w 100 | tee final-result | cat
printf "\n"

# Generate checksum for shadow array
CHECKSUM=0
for (( i=0; i<${#SHADOW_ARRAY[@]}; i++ ))
do
  if [[ ${SHADOW_ARRAY[i]} -ne -1 ]] ;then (( CHECKSUM += i * SHADOW_ARRAY[i] )); fi
done
printf "%s\n" "$CHECKSUM"