1 files changed, 185 insertions, 0 deletions
diff --git a/src/app_state.rs b/src/app_state.rs
new file mode 100644
index 0000000..4a0912e
--- /dev/null
+++ b/src/app_state.rs
@@ -0,0 +1,185 @@
+use anyhow::{bail, Context, Result};
+use serde::{Deserialize, Serialize};
+use std::fs;
+
+use crate::exercise::Exercise;
+
+const BAD_INDEX_ERR: &str = "The current exercise index is higher than the number of exercises";
+
+#[derive(Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+struct StateFile {
+    current_exercise_ind: usize,
+    progress: Vec<bool>,
+}
+
+impl StateFile {
+    fn read(exercises: &[Exercise]) -> Option<Self> {
+        let file_content = fs::read(".rustlings-state.json").ok()?;
+
+        let slf: Self = serde_json::de::from_slice(&file_content).ok()?;
+
+        if slf.progress.len() != exercises.len() || slf.current_exercise_ind >= exercises.len() {
+            return None;
+        }
+
+        Some(slf)
+    }
+
+    fn read_or_default(exercises: &[Exercise]) -> Self {
+        Self::read(exercises).unwrap_or_else(|| Self {
+            current_exercise_ind: 0,
+            progress: vec![false; exercises.len()],
+        })
+    }
+
+    fn write(&self) -> Result<()> {
+        let mut buf = Vec::with_capacity(1024);
+        serde_json::ser::to_writer(&mut buf, self).context("Failed to serialize the state")?;
+        fs::write(".rustlings-state.json", buf)
+            .context("Failed to write the state file `.rustlings-state.json`")?;
+
+        Ok(())
+    }
+}
+
+pub struct AppState {
+    state_file: StateFile,
+    exercises: &'static [Exercise],
+    n_done: u16,
+    current_exercise: &'static Exercise,
+}
+
+#[must_use]
+pub enum ExercisesProgress {
+    AllDone,
+    Pending,
+}
+
+impl AppState {
+    pub fn new(exercises: Vec<Exercise>) -> Self {
+        // Leaking for sending the exercises to the debounce event handler.
+        // Leaking is not a problem since the exercises' slice is used until the end of the program.
+        let exercises = exercises.leak();
+
+        let state_file = StateFile::read_or_default(exercises);
+        let n_done = state_file
+            .progress
+            .iter()
+            .fold(0, |acc, done| acc + u16::from(*done));
+        let current_exercise = &exercises[state_file.current_exercise_ind];
+
+        Self {
+            state_file,
+            exercises,
+            n_done,
+            current_exercise,
+        }
+    }
+
+    #[inline]
+    pub fn current_exercise_ind(&self) -> usize {
+        self.state_file.current_exercise_ind
+    }
+
+    #[inline]
+    pub fn progress(&self) -> &[bool] {
+        &self.state_file.progress
+    }
+
+    #[inline]
+    pub fn exercises(&self) -> &'static [Exercise] {
+        self.exercises
+    }
+
+    #[inline]
+    pub fn n_done(&self) -> u16 {
+        self.n_done
+    }
+
+    #[inline]
+    pub fn current_exercise(&self) -> &'static Exercise {
+        self.current_exercise
+    }
+
+    pub fn set_current_exercise_ind(&mut self, ind: usize) -> Result<()> {
+        if ind >= self.exercises.len() {
+            bail!(BAD_INDEX_ERR);
+        }
+
+        self.state_file.current_exercise_ind = ind;
+        self.current_exercise = &self.exercises[ind];
+
+        self.state_file.write()
+    }
+
+    pub fn set_current_exercise_by_name(&mut self, name: &str) -> Result<()> {
+        let (ind, exercise) = self
+            .exercises
+            .iter()
+            .enumerate()
+            .find(|(_, exercise)| exercise.name == name)
+            .with_context(|| format!("No exercise found for '{name}'!"))?;
+
+        self.state_file.current_exercise_ind = ind;
+        self.current_exercise = exercise;
+
+        self.state_file.write()
+    }
+
+    pub fn set_pending(&mut self, ind: usize) -> Result<()> {
+        let done = self
+            .state_file
+            .progress
+            .get_mut(ind)
+            .context(BAD_INDEX_ERR)?;
+
+        if *done {
+            *done = false;
+            self.n_done -= 1;
+            self.state_file.write()?;
+        }
+
+        Ok(())
+    }
+
+    fn next_exercise_ind(&self) -> Option<usize> {
+        let current_ind = self.state_file.current_exercise_ind;
+
+        if current_ind == self.state_file.progress.len() - 1 {
+            // The last exercise is done.
+            // Search for exercises not done from the start.
+            return self.state_file.progress[..current_ind]
+                .iter()
+                .position(|done| !done);
+        }
+
+        // The done exercise isn't the last one.
+        // Search for a pending exercise after the current one and then from the start.
+        match self.state_file.progress[current_ind + 1..]
+            .iter()
+            .position(|done| !done)
+        {
+            Some(ind) => Some(current_ind + 1 + ind),
+            None => self.state_file.progress[..current_ind]
+                .iter()
+                .position(|done| !done),
+        }
+    }
+
+    pub fn done_current_exercise(&mut self) -> Result<ExercisesProgress> {
+        let done = &mut self.state_file.progress[self.state_file.current_exercise_ind];
+        if !*done {
+            *done = true;
+            self.n_done += 1;
+        }
+
+        let Some(ind) = self.next_exercise_ind() else {
+            return Ok(ExercisesProgress::AllDone);
+        };
+
+        self.set_current_exercise_ind(ind)?;
+
+        Ok(ExercisesProgress::Pending)
+    }
+}