Calculate differences for phases

Hello,

I have an experiment where I had participants (identified individually with ID) from 4 different groups eat 6 different snacks and then rate them. There were 4 phases/repetitions.


Now I want to calculate the differences between the phases to check whether there are significant intra-individual differences.
Finally, I will want to calculate the significance with an ANOVA and also visually plot the differences. Because the ANOVA needs long format, I wanted to add this information.
I want an additional column at the back with those differences, i.e. difference from phase 2 to phase 1, from phase 3 to phase 1, and from phase 4 to phase 1. And an additional column that states what is encoded there, i.e. which phase difference value is in the column in front of it.

I know this looks ridiculous but just concerning the structure, might something like this be possible:

add new columns diff_ratings (and diff_phases) to table_liking
for (each line until the end, check cloumn:Phase)
   if Phase=1
      Rating of this line -(MINUS) Rating of this line of same ID and Snack
      und write the value in the new column diff_ratings
      (and write ph1-ph1 in the other new column)
   if Phase=2
      Rating of this line - Rating of this line of same ID and Snack from Phase 1
      und write the value in the new column diff_ratings
      (and write ph2-ph1 in the other new column)
   if Phase=3
      Rating of this line - Rating of this line of same ID and Snack from Phase 1
      und write the value in the new column diff_ratings
      (and write ph3-ph1 in the other new column)
   if Phase=4
      Rating of this line - Rating of this line of same ID and Snack from Phase 1
      und write the value in the new column diff_ratings
      (and write ph4-ph1 in the other new column)

I am having a hard time wrapping my head around even theoretically understanding it myself, so apologies in advance for my attempt at an explanation.
If you have any idea but need aditional information please ask.
Thank you!


maybe this sketch can help to clarify my idea/problem