diff --git a/src-tauri/src/azookey/azookey_conversion.rs b/src-tauri/src/azookey/azookey_conversion.rs
index c96d5a4..589bfd5 100644
--- a/src-tauri/src/azookey/azookey_conversion.rs
+++ b/src-tauri/src/azookey/azookey_conversion.rs
@@ -1,241 +1,423 @@
-use anyhow::Result;
-use tracing::{debug, error, info, trace};
+use anyhow::{anyhow, Result};
+use tracing::{debug, info, trace};
 
 use super::client::AzookeyConversionClient;
 
+/// Maximum number of history entries to retain
+const MAX_HISTORY_SIZE: usize = 3;
+
+/// Maximum number of conversion candidates
+const MAX_CANDIDATES: usize = 10;
+
+/// AzookeyConversion - Provides romanized text to kanji conversion and candidate switching
+///
+/// This struct implements the logic for character conversion and candidate switching
+/// in a Japanese input method system.
 pub struct AzookeyConversion {
-    pub conversion_history: Vec<String>,
-    pub clipboard_history: Vec<String>,
-    pub now_reconvertion: bool,
-    pub target_text: String,
-    pub reconversion_candidates: Option<Vec<String>>,
-    pub reconversion_index: Option<i32>,
-    pub reconversion_prefix: Option<String>,
-    pub client: AzookeyConversionClient,
+    /// Conversion history (max 3 entries)
+    conversion_history: Vec<String>,
+
+    /// Pre-conversion text history (max 3 entries)
+    input_history: Vec<String>,
+
+    /// Whether currently in reconversion mode
+    is_reconversion_mode: bool,
+
+    /// Current text being converted
+    current_text: String,
+
+    /// List of reconversion candidates
+    reconversion_candidates: Option<Vec<String>>,
+
+    /// Index of currently selected reconversion candidate
+    candidate_index: Option<usize>,
+
+    /// Common prefix for reconversion
+    common_prefix: Option<String>,
+
+    /// Client that performs conversion operations
+    client: AzookeyConversionClient,
 }
 
 impl AzookeyConversion {
+    /// Creates a new AzookeyConversion instance
+    ///
+    /// # Arguments
+    /// * `client` - Client instance that performs conversion operations
+    ///
+    /// # Returns
+    /// * Initialized AzookeyConversion instance
     pub fn new(client: AzookeyConversionClient) -> Self {
         info!("Creating new AzookeyConversion instance");
 
-        let instance = Self {
+        Self {
             conversion_history: Vec::new(),
-            clipboard_history: Vec::new(),
-            now_reconvertion: false,
-            target_text: String::new(),
+            input_history: Vec::new(),
+            is_reconversion_mode: false,
+            current_text: String::new(),
             reconversion_candidates: None,
-            reconversion_index: None,
-            reconversion_prefix: None,
+            candidate_index: None,
+            common_prefix: None,
             client,
-        };
-        instance
-    }
-
-    fn reset_conversion_state(&mut self) {
-        debug!("Resetting conversion state");
-        trace!("Before reset - now_reconvertion: {}, reconversion_prefix: {:?}, reconversion_index: {:?}, reconversion_candidates: {:?}",
-               self.now_reconvertion, self.reconversion_prefix, self.reconversion_index, self.reconversion_candidates);
-        self.now_reconvertion = false;
-        self.reconversion_prefix = None;
-        self.reconversion_index = None;
-        self.reconversion_candidates = None;
-        trace!("After reset - now_reconvertion: {}, reconversion_prefix: {:?}, reconversion_index: {:?}, reconversion_candidates: {:?}",
-               self.now_reconvertion, self.reconversion_prefix, self.reconversion_index, self.reconversion_candidates);
-    }
-
-    fn convert_roman_to_kanji(&mut self, text: &str) -> Result<String> {
-        debug!("Converting roman to kanji: {}", text);
-        let o_minus_1 = self
-            .conversion_history
-            .get(if self.conversion_history.len() > 0 {
-                self.conversion_history.len() - 1
-            } else {
-                0
-            })
-            .unwrap_or(&("".to_string()))
-            .clone();
-        trace!("Previous conversion (o_minus_1): {}", o_minus_1);
-        let mut first_diff_position = o_minus_1
-            .chars()
-            .zip(text.chars())
-            .position(|(a, b)| a != b);
-
-        if o_minus_1 != text && first_diff_position.is_none() {
-            first_diff_position = Some(o_minus_1.chars().count());
-        }
-        trace!("First difference position: {:?}", first_diff_position);
-        let diff = text
-            .chars()
-            .skip(first_diff_position.unwrap_or(0))
-            .collect::<String>();
-        debug!("Difference to convert: {}", diff);
-
-        let prefix = text
-            .chars()
-            .take(first_diff_position.unwrap_or(0))
-            .collect::<String>();
-        trace!("Prefix for conversion: {}", prefix);
-        self.client.reset_composing_text();
-        self.client.insert_at_cursor_position(&diff);
-        trace!("target: {}, prefix: {}", diff, prefix);
-        let converted = self
-            .client
-            .request_candidates("")
-            .first()
-            .unwrap()
-            .text
-            .clone();
-        trace!("Converted difference: {}", converted);
-        self.conversion_history.push(
-            o_minus_1
-                .chars()
-                .zip(text.chars())
-                .take_while(|(a, b)| a == b)
-                .map(|(a, _)| a)
-                .collect::<String>()
-                + &converted,
-        );
-        self.clipboard_history.push(text.to_string());
-        info!(
-            "Roman to kanji conversion result: {}",
-            self.conversion_history.last().unwrap()
-        );
-        trace!("Updated conversion history: {:?}", self.conversion_history);
-        trace!("Updated clipboard history: {:?}", self.clipboard_history);
-        Ok(self.conversion_history.last().unwrap().clone())
-    }
-
-    fn convert_tsf(&mut self, text: &str) -> Result<String> {
-        debug!("Converting using TSF: {}", text);
-        self.now_reconvertion = true;
-        let mut diff = String::new();
-        if self.reconversion_prefix.is_none() {
-            let o_minus_2 = self
-                .conversion_history
-                .get(if self.conversion_history.len() > 1 {
-                    self.conversion_history.len() - 2
-                } else {
-                    0
-                })
-                .unwrap_or(&("".to_string()))
-                .clone();
-            let i_minus_1 = self
-                .clipboard_history
-                .get(if self.clipboard_history.len() > 0 {
-                    self.clipboard_history.len() - 1
-                } else {
-                    0
-                })
-                .unwrap_or(&("".to_string()))
-                .clone();
-            trace!("o_minus_2: {}, i_minus_1: {}", o_minus_2, i_minus_1);
-            let mut first_diff_position = i_minus_1
-                .chars()
-                .zip(o_minus_2.chars())
-                .position(|(a, b)| a != b);
-            trace!("First difference position: {:?}", first_diff_position);
-            if o_minus_2 != i_minus_1 && first_diff_position.is_none() {
-                first_diff_position = Some(o_minus_2.chars().count());
-            }
-            diff = i_minus_1
-                .chars()
-                .skip(first_diff_position.unwrap_or(0))
-                .collect::<String>();
-            debug!("Difference to convert: {}", diff);
-            let prefix = i_minus_1
-                .chars()
-                .zip(o_minus_2.chars())
-                .take_while(|(a, b)| a == b)
-                .map(|(a, _)| a)
-                .collect::<String>();
-            self.reconversion_prefix = Some(prefix.clone());
-            trace!("Set reconversion prefix: {:?}", self.reconversion_prefix);
-        }
-
-        let candidates = self.reconversion_candidates.get_or_insert_with(|| {
-            debug!("Generating new candidates");
-            self.client.reset_composing_text();
-            self.client.insert_at_cursor_position(&diff);
-            let prefix = self.reconversion_prefix.clone().unwrap_or_default();
-            let mut candidates = self
-                .client
-                .request_candidates(&prefix)
-                .iter()
-                .map(|c| c.text.clone())
-                .collect::<Vec<String>>();
-            trace!("Initial candidates: {:?}", candidates);
-            candidates.insert(0, diff.to_string());
-            trace!("Final candidates: {:?}", candidates);
-            candidates
-        });
-
-        let index = self.reconversion_index.get_or_insert(-1);
-        trace!("Current reconversion index: {}", index);
-
-        if *index + 1 < candidates.len() as i32 {
-            *index += 1;
-        } else {
-            *index = 0;
-        }
-        debug!("Updated reconversion index: {}", index);
-
-        self.conversion_history.push(
-            self.reconversion_prefix.clone().unwrap()
-                + &self.reconversion_candidates.as_ref().unwrap()
-                    [self.reconversion_index.unwrap() as usize]
-                    .clone(),
-        );
-        self.clipboard_history.push(text.to_string());
-        trace!("Updated conversion history: {:?}", self.conversion_history);
-        trace!("Updated clipboard history: {:?}", self.clipboard_history);
-
-        while self.conversion_history.len() > 3 {
-            self.conversion_history.remove(0);
-        }
-        while self.clipboard_history.len() > 3 {
-            self.clipboard_history.remove(0);
-        }
-        trace!("Trimmed conversion history: {:?}", self.conversion_history);
-        trace!("Trimmed clipboard history: {:?}", self.clipboard_history);
-
-        info!(
-            "TSF conversion result: {}",
-            self.conversion_history.last().unwrap()
-        );
-        Ok(self.conversion_history.last().unwrap().clone())
+        }
     }
 
+    /// Converts text - Main entry point for conversion processing
+    ///
+    /// # Arguments
+    /// * `text` - Text to be converted
+    ///
+    /// # Returns
+    /// * `Result<String>` - Conversion result or error
     pub fn convert(&mut self, text: &str) -> Result<String> {
-        debug!("Starting conversion for: {}", text);
-        trace!("Current conversion history: {:?}", self.conversion_history);
-        trace!("Current clipboard history: {:?}", self.clipboard_history);
-        let same_as_last_conversion = text.to_string()
-            == self
-                .conversion_history
-                .last()
-                .unwrap_or(&("".to_string()))
-                .clone();
+        debug!("Starting conversion: {}", text);
+        trace!(
+            "Current state: conversion_history={:?}, input_history={:?}, is_reconversion_mode={}",
+            self.conversion_history,
+            self.input_history,
+            self.is_reconversion_mode
+        );
+
+        self.current_text = text.to_string();
+
+        // Check if same as previous conversion result
+        let same_as_last_conversion = self.is_same_as_last_conversion(text);
         trace!("Same as last conversion: {}", same_as_last_conversion);
 
-        self.target_text = text.to_string();
-        trace!("Set target text: {}", self.target_text);
-
-        if !same_as_last_conversion && self.now_reconvertion {
+        // Reset if input changed while in reconversion mode
+        if !same_as_last_conversion && self.is_reconversion_mode {
             debug!("Resetting conversion state due to new input");
-            self.reset_conversion_state();
+            self.reset_reconversion_state();
         }
 
-        if !self.now_reconvertion && !same_as_last_conversion {
-            info!("Converting using roman_to_kanji");
-            return self.convert_roman_to_kanji(text);
+        // Branch conversion processing
+        if self.is_reconversion_mode || same_as_last_conversion {
+            // Same text re-entered or in reconversion mode
+            info!("Executing AzooKey conversion");
+            self.convert_with_candidates(text)
+        } else {
+            // Normal conversion processing
+            info!("Executing regular romaji->kanji conversion");
+            self.convert_roman_to_kanji(text)
+        }
+    }
+
+    /// Determines if input is the same as the last conversion result
+    ///
+    /// # Arguments
+    /// * `text` - Text to check
+    ///
+    /// # Returns
+    /// * `bool` - True if same as last conversion
+    fn is_same_as_last_conversion(&self, text: &str) -> bool {
+        if let Some(last_conversion) = self.conversion_history.last() {
+            text == last_conversion
+        } else {
+            false
+        }
+    }
+
+    /// Resets reconversion-related state
+    fn reset_reconversion_state(&mut self) {
+        debug!("Resetting reconversion state");
+
+        trace!(
+            "Before reset - is_reconversion_mode: {}, common_prefix: {:?}, candidate_index: {:?}",
+            self.is_reconversion_mode,
+            self.common_prefix,
+            self.candidate_index
+        );
+
+        self.is_reconversion_mode = false;
+        self.common_prefix = None;
+        self.candidate_index = None;
+        self.reconversion_candidates = None;
+
+        trace!(
+            "After reset - is_reconversion_mode: {}, common_prefix: {:?}, candidate_index: {:?}",
+            self.is_reconversion_mode,
+            self.common_prefix,
+            self.candidate_index
+        );
+    }
+
+    /// Converts romaji to kanji (initial conversion)
+    ///
+    /// # Arguments
+    /// * `text` - Text to convert
+    ///
+    /// # Returns
+    /// * `Result<String>` - Conversion result or error
+    fn convert_roman_to_kanji(&mut self, text: &str) -> Result<String> {
+        debug!("Converting romaji to kanji: {}", text);
+
+        // Get previous conversion result
+        let previous_conversion = self.get_previous_conversion();
+        trace!("Previous conversion: {}", previous_conversion);
+
+        // Detect difference position
+        let first_diff_position = self.find_first_difference(&previous_conversion, text);
+        trace!("First difference position: {:?}", first_diff_position);
+
+        // Extract difference text
+        let diff_text = text.chars().skip(first_diff_position).collect::<String>();
+        debug!("Difference to convert: {}", diff_text);
+
+        // Extract common prefix
+        let prefix = text.chars().take(first_diff_position).collect::<String>();
+        trace!("Conversion prefix: {}", prefix);
+
+        // Use client for conversion
+        self.client.reset_composing_text();
+        self.client.insert_at_cursor_position(&diff_text);
+
+        // Get and select conversion candidate
+        let converted = match self.client.request_candidates("").first() {
+            Some(candidate) => candidate.text.clone(),
+            None => return Err(anyhow!("No conversion candidates available")),
+        };
+        trace!("Conversion result: {}", converted);
+
+        // Update history
+        let result = prefix + &converted;
+        self.update_history(result.clone(), text.to_string());
+
+        info!("Romaji->kanji conversion result: {}", result);
+        Ok(result)
+    }
+
+    /// Switches between conversion candidates (reconversion)
+    ///
+    /// # Arguments
+    /// * `text` - Text for reconversion
+    ///
+    /// # Returns
+    /// * `Result<String>` - Conversion result or error
+    fn convert_with_candidates(&mut self, text: &str) -> Result<String> {
+        debug!("Converting with AzooKey: {}", text);
+        self.is_reconversion_mode = true;
+
+        // Prepare for reconversion if needed
+        self.prepare_reconversion_if_needed();
+
+        // Select and switch candidates
+        let result = self.select_next_candidate()?;
+
+        // Update history
+        self.update_history(result.clone(), text.to_string());
+
+        info!("AzooKey conversion result: {}", result);
+        Ok(result)
+    }
+
+    /// Prepares difference processing for reconversion
+    fn prepare_reconversion_if_needed(&mut self) {
+        // Only execute on first reconversion
+        if self.common_prefix.is_none() {
+            debug!("Preparing for reconversion");
+
+            // Calculate differences from past history
+            let previous_output = self.get_previous_output(2);
+            let previous_input = self.get_previous_input(1);
+            trace!(
+                "Previous output: {}, previous input: {}",
+                previous_output,
+                previous_input
+            );
+
+            // Detect difference position
+            let first_diff_position = self.find_first_difference(&previous_input, &previous_output);
+            trace!("First difference position: {:?}", first_diff_position);
+
+            // Extract difference text
+            let diff_text = previous_input
+                .chars()
+                .skip(first_diff_position)
+                .collect::<String>();
+            debug!("Reconversion difference: {}", diff_text);
+
+            // Set common prefix
+            let prefix = previous_input
+                .chars()
+                .take(first_diff_position)
+                .collect::<String>();
+            self.common_prefix = Some(prefix.clone());
+            trace!("Set reconversion prefix: {}", prefix);
+
+            // Generate candidates
+            self.generate_candidates(&diff_text, &prefix);
+        }
+    }
+
+    /// Generates conversion candidates
+    ///
+    /// # Arguments
+    /// * `diff_text` - Difference text to convert
+    /// * `prefix` - Common prefix
+    fn generate_candidates(&mut self, diff_text: &str, prefix: &str) {
+        debug!("Generating candidates");
+
+        self.client.reset_composing_text();
+        self.client.insert_at_cursor_position(diff_text);
+
+        // Get candidates from client
+        let mut candidates = self
+            .client
+            .request_candidates(prefix)
+            .iter()
+            .map(|c| c.text.clone())
+            .collect::<Vec<String>>();
+        trace!("Retrieved candidates: {:?}", candidates);
+
+        // Include raw text in candidates
+        candidates.insert(0, diff_text.to_string());
+
+        // Limit number of candidates (for safety)
+        if candidates.len() > MAX_CANDIDATES {
+            candidates.truncate(MAX_CANDIDATES);
         }
 
-        if same_as_last_conversion || self.now_reconvertion {
-            info!("Converting using TSF");
-            return self.convert_tsf(text);
+        trace!("Final candidate list: {:?}", candidates);
+        self.reconversion_candidates = Some(candidates);
+        self.candidate_index = Some(0);
+    }
+
+    /// Selects the next candidate
+    ///
+    /// # Returns
+    /// * `Result<String>` - Selected candidate or error
+    fn select_next_candidate(&mut self) -> Result<String> {
+        let candidates = match &self.reconversion_candidates {
+            Some(cands) => cands,
+            None => return Err(anyhow!("Candidate list does not exist")),
+        };
+
+        let index = match self.candidate_index {
+            Some(i) => {
+                // Update index
+                let new_index = if i + 1 < candidates.len() { i + 1 } else { 0 };
+                self.candidate_index = Some(new_index);
+                new_index
+            }
+            None => return Err(anyhow!("Candidate index not initialized")),
+        };
+
+        debug!("Updated candidate index: {}", index);
+
+        // Get selected candidate
+        let prefix = self.common_prefix.clone().unwrap_or_default();
+        let selected_candidate = &candidates[index];
+        let result = prefix + selected_candidate;
+
+        Ok(result)
+    }
+
+    /// Updates history
+    ///
+    /// # Arguments
+    /// * `conversion` - Conversion result
+    /// * `input` - Input text
+    fn update_history(&mut self, conversion: String, input: String) {
+        self.conversion_history.push(conversion);
+        self.input_history.push(input);
+
+        trace!(
+            "Before trim: conversion_history={}, input_history={}",
+            self.conversion_history.len(),
+            self.input_history.len()
+        );
+
+        // Limit history size
+        self.trim_history();
+
+        trace!(
+            "After trim: conversion_history={}, input_history={}",
+            self.conversion_history.len(),
+            self.input_history.len()
+        );
+    }
+
+    /// Limits history size
+    fn trim_history(&mut self) {
+        while self.conversion_history.len() > MAX_HISTORY_SIZE {
+            self.conversion_history.remove(0);
         }
 
-        error!("Failed to convert: {}", text);
-        Err(anyhow::anyhow!("Failed to convert"))
+        while self.input_history.len() > MAX_HISTORY_SIZE {
+            self.input_history.remove(0);
+        }
+    }
+
+    /// Gets previous conversion result
+    ///
+    /// # Returns
+    /// * `String` - Previous conversion result or empty string
+    fn get_previous_conversion(&self) -> String {
+        self.conversion_history
+            .last()
+            .map(|s| s.clone())
+            .unwrap_or_default()
+    }
+
+    /// Gets conversion history from specified index
+    ///
+    /// # Arguments
+    /// * `offset` - Offset from the end
+    ///
+    /// # Returns
+    /// * `String` - Retrieved history or empty string
+    fn get_previous_output(&self, offset: usize) -> String {
+        if offset <= self.conversion_history.len() {
+            self.conversion_history[self.conversion_history.len() - offset].clone()
+        } else {
+            String::new()
+        }
+    }
+
+    /// Gets input history from specified index
+    ///
+    /// # Arguments
+    /// * `offset` - Offset from the end
+    ///
+    /// # Returns
+    /// * `String` - Retrieved history or empty string
+    fn get_previous_input(&self, offset: usize) -> String {
+        if offset <= self.input_history.len() {
+            self.input_history[self.input_history.len() - offset].clone()
+        } else {
+            String::new()
+        }
+    }
+
+    /// Finds first difference position between two strings
+    ///
+    /// # Arguments
+    /// * `s1` - First string to compare
+    /// * `s2` - Second string to compare
+    ///
+    /// # Returns
+    /// * `usize` - First difference position
+    fn find_first_difference(&self, s1: &str, s2: &str) -> usize {
+        let result = s1
+            .chars()
+            .zip(s2.chars())
+            .position(|(a, b)| a != b)
+            .unwrap_or_else(|| {
+                // If one is a prefix of the other
+                let min_len = s1.chars().count().min(s2.chars().count());
+                if s1.len() != s2.len() {
+                    min_len
+                } else {
+                    0
+                }
+            });
+
+        trace!(
+            "String comparison: \"{}\" and \"{}\" differ at position: {}",
+            s1,
+            s2,
+            result
+        );
+        result
     }
 }