diff --git a/.github/workflows/r.yml b/.github/workflows/r.yml
index 10933c2..a8393f5 100644
--- a/.github/workflows/r.yml
+++ b/.github/workflows/r.yml
@@ -33,6 +33,7 @@ on:
       - main
     paths:
       - "R/**"
+      - "tests/**"
       - ".github/workflows/r.yml"
 
 jobs:
@@ -78,14 +79,17 @@ jobs:
       - name: Install R dependencies
         run: |
           Rscript -e '
-          packages <- c("httr", "jsonlite", "dplyr", "magrittr", "showtext", "ggplot2", "treemapify", "svglite")
+          packages <- c("httr", "jsonlite", "dplyr", "showtext", "sysfonts", "ggplot2", "treemapify", "svglite")
           new_packages <- packages[!(packages %in% installed.packages()[,"Package"])]
           if(length(new_packages)) {
-            install.packages(new_packages, repos="https://cloud.r-project.org", dependencies=TRUE)
+            install.packages(new_packages, repos="https://cloud.r-project.org", dependencies=NA)
           }
           '
         shell: bash
 
+      - name: Run tests
+        run: Rscript tests/run_tests.R
+
       - name: Run analysis
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
diff --git a/R/fetch.R b/R/fetch.R
index 8423576..5955473 100644
--- a/R/fetch.R
+++ b/R/fetch.R
@@ -12,17 +12,6 @@
 # Last Modified: 2025-10-25 17:25:34
 ##
 
-# Load required packages
-if (!requireNamespace("httr", quietly = TRUE)) {
-  install.packages("httr")
-}
-library(httr)
-
-if (!requireNamespace("jsonlite", quietly = TRUE)) {
-  install.packages("jsonlite")
-}
-library(jsonlite)
-
 #' Fetch GitHub Repository Data
 #'
 #' Fetches repository information for a GitHub user via the GitHub API.
@@ -65,20 +54,20 @@ fetch_json <- function(username, proxy = NULL) {
       }
 
       # Make HTTP request
-      response <- GET(
+      response <- httr::GET(
         url,
-        add_headers(.headers = headers),
-        if (!is.null(proxy)) use_proxy(proxy)
+        httr::add_headers(.headers = headers),
+        if (!is.null(proxy)) httr::use_proxy(proxy)
       )
 
       # Check response status
-      if (status_code(response) != 200) {
-        stop("HTTP request failed with status code: ", status_code(response))
+      if (httr::status_code(response) != 200) {
+        stop("HTTP request failed with status code: ", httr::status_code(response))
       }
 
       # Parse JSON response
-      content_text <- content(response, "text", encoding = "UTF-8")
-      return(fromJSON(content_text))
+      content_text <- httr::content(response, "text", encoding = "UTF-8")
+      return(jsonlite::fromJSON(content_text))
     },
     error = function(e) {
       message("Error fetching JSON data: ", e$message)
diff --git a/R/fmt.R b/R/fmt.R
index 0384dc0..15b975b 100644
--- a/R/fmt.R
+++ b/R/fmt.R
@@ -12,9 +12,8 @@
 # Last Modified: 2025-10-25 23:57:46
 ##
 
-if (!require("styler", quietly = TRUE)) {
-  install.packages("styler")
-  library(styler)
+if (!requireNamespace("styler", quietly = TRUE)) {
+  stop("Missing required package: styler. Install it with install.packages().")
 }
 
 #' Format R Code Files
diff --git a/R/helpers.R b/R/helpers.R
new file mode 100644
index 0000000..14e0a8e
--- /dev/null
+++ b/R/helpers.R
@@ -0,0 +1,20 @@
+setup_visualization_font <- function(font_path = file.path("assets", "FiraCode.ttf")) {
+  font_family <- "sans"
+
+  if (!file.exists(font_path)) {
+    warning("Custom font not found at ", font_path, "; using default font.")
+    return(font_family)
+  }
+
+  tryCatch(
+    {
+      sysfonts::font_add("FiraCode", regular = font_path)
+      showtext::showtext_auto()
+      "FiraCode"
+    },
+    error = function(error) {
+      warning("Failed to load custom font, using default: ", error$message)
+      font_family
+    }
+  )
+}
diff --git a/R/main.R b/R/main.R
index b12ed93..96a1afd 100644
--- a/R/main.R
+++ b/R/main.R
@@ -14,23 +14,31 @@
 # Last Modified: 2025-10-25 16:18:16
 ##
 
-# Load required packages
-if (!require("jsonlite", quietly = TRUE)) {
-  install.packages("jsonlite")
-  library(jsonlite)
+required_packages <- c(
+  "dplyr", "ggplot2", "httr", "jsonlite", "showtext", "svglite", "sysfonts",
+  "treemapify"
+)
+missing_packages <- required_packages[
+  !vapply(required_packages, requireNamespace, logical(1), quietly = TRUE)
+]
+if (length(missing_packages) > 0) {
+  stop(
+    "Missing required packages: ", paste(missing_packages, collapse = ", "),
+    ". Install them with install.packages()."
+  )
 }
 
+source_module <- function(path) {
+  tryCatch(
+    suppressMessages(source(path)),
+    error = function(error) stop("Failed to source ", path, ": ", error$message)
+  )
+}
 
-# Source helper functions
-tryCatch(
-  suppressMessages(source("R/fetch.R")),
-  error = function(e) stop("Failed to source R/fetch.R: ", e$message)
-)
-
-tryCatch(
-  suppressMessages(source("R/plot.R")),
-  error = function(e) stop("Failed to source R/plot.R: ", e$message)
-)
+invisible(lapply(
+  c("R/helpers.R", "R/fetch.R", "R/plot.R", "R/pie.R", "R/treemap.R"),
+  source_module
+))
 
 # Get GitHub username from environment or use default
 username <- Sys.getenv("GITHUB_USERNAME", unset = "mingcheng")
@@ -50,12 +58,12 @@ plotted_file_name <- file.path(dir_name, paste0("plotted_", today, ".csv"))
 
 # Fetch or load cached repository data
 if (file.exists(raw_file_name)) {
-  json_data <- fromJSON(raw_file_name)
+  json_data <- jsonlite::fromJSON(raw_file_name)
   message("Loaded cached data from ", raw_file_name)
 } else {
   # Check for HTTP proxy settings
   proxy <- Sys.getenv("HTTP_PROXY", unset = "")
-  if (is.null(proxy) || proxy == "") {
+  if (!nzchar(proxy)) {
     proxy <- NULL
     message("No proxy configured.")
   } else {
@@ -68,7 +76,7 @@ if (file.exists(raw_file_name)) {
   }
   message("Fetched ", length(json_data), " repositories")
 
-  write_json(json_data, raw_file_name, pretty = TRUE, auto_unbox = TRUE)
+  jsonlite::write_json(json_data, raw_file_name, pretty = TRUE, auto_unbox = TRUE)
   message("Raw data saved to ", raw_file_name)
 }
 
@@ -83,25 +91,8 @@ message("Successfully processed repository data")
 write.csv(repos, plotted_file_name, row.names = FALSE)
 message("Statistical data saved to ", plotted_file_name)
 
-# Generate visualizations
-
-# Generate pie chart
-tryCatch(
-  suppressMessages(source("R/pie.R")),
-  error = function(e) stop("Failed to source R/pie.R: ", e$message)
-)
-
 generate_pie_chart(username, repos, dir_name, json_data)
 
-# Generate treemap
-tryCatch(
-  suppressMessages(source("R/treemap.R")),
-  error = function(e) stop("Failed to source R/treemap.R: ", e$message)
-)
-
 generate_treemap(username, repos, dir_name, json_data)
 
-
-
-
 message("Analysis complete!")
diff --git a/R/pie.R b/R/pie.R
index c11f2b9..7bc2462 100644
--- a/R/pie.R
+++ b/R/pie.R
@@ -12,22 +12,8 @@
 # Last Modified: 2025-10-26 11:19:07
 ##
 
-if (!require("showtext", quietly = TRUE)) {
-  install.packages("showtext")
-  library(showtext)
-}
-
 generate_pie_chart <- function(username, repos, dir_name, json_data) {
-  # Setup font for visualization
-  tryCatch(
-    font_add("FiraCode", regular = "../assets/FiraCode.ttf"),
-    error = function(e) {
-      warning("Failed to load custom font, using default: ", e$message)
-    }
-  )
-
-  # Enable showtext before opening graphics device
-  showtext_auto()
+  font_family <- setup_visualization_font()
 
   # Define function to generate chart
   generate_chart <- function(username, repos, colors, total_repos, date_str) {
@@ -35,7 +21,7 @@ generate_pie_chart <- function(username, repos, dir_name, json_data) {
       bg = "white",
       mai = c(0.5, 0.5, 0.8, 0.5),
       lwd = 1,
-      family = "FiraCode"
+      family = font_family
     )
 
     pie(
@@ -58,6 +44,7 @@ generate_pie_chart <- function(username, repos, dir_name, json_data) {
   colors <- rainbow(nrow(repos))
   total_repos <- nrow(json_data)
   date_str <- format(Sys.Date(), "%Y-%m-%d")
+  today <- format(Sys.Date(), "%Y%m%d")
 
   # Define chart files
   chart_files <- list(
diff --git a/R/plot.R b/R/plot.R
index e778255..d404d80 100644
--- a/R/plot.R
+++ b/R/plot.R
@@ -12,22 +12,6 @@
 # Last Modified: 2025-10-25 17:08:48
 ##
 
-# Load required packages
-if (!require("dplyr", quietly = TRUE)) {
-  install.packages("dplyr")
-  library(dplyr)
-}
-
-if (!require("magrittr", quietly = TRUE)) {
-  install.packages("magrittr")
-  library(magrittr)
-}
-
-if (!require("magrittr", quietly = TRUE)) {
-  install.packages("magrittr")
-  library(magrittr)
-}
-
 #' Process and Analyze Repository Data
 #'
 #' Calculate weighted language distribution scores based on repository
@@ -44,8 +28,8 @@ plot_repos <- function(repos_data) {
   cat("\n")
 
   # Calculate weighted scores and aggregate by language
-  repos_df <- repos_data %>%
-    mutate(
+  repos_df <- repos_data |>
+    dplyr::mutate(
       # Calculate update recency weight (most important factor)
       days_since_update = as.numeric(Sys.Date() - as.Date(updated_at)),
       # Exponential decay: 1 year decay constant
@@ -56,27 +40,27 @@ plot_repos <- function(repos_data) {
       # Weighted score: priority = update time > repo count > stars > forks
       # Note: repo count is calculated after grouping
       weighted_score = (star_score * 0.2 + fork_score * 0.1 + 1.5) * (date_weight^1.5)
-    ) %>%
+    ) |>
     # Group by language and aggregate
-    group_by(language) %>%
-    summarise(
+    dplyr::group_by(language) |>
+    dplyr::summarise(
       total_score = sum(weighted_score, na.rm = TRUE),
-      repo_count = n(),
+      repo_count = dplyr::n(),
       total_stars = sum(stargazers_count, na.rm = TRUE),
       total_forks = sum(forks_count, na.rm = TRUE),
       avg_days_since_update = mean(days_since_update, na.rm = TRUE),
       avg_date_weight = mean(date_weight, na.rm = TRUE),
       .groups = "drop"
-    ) %>%
+    ) |>
     # Filter out NA languages
-    filter(!is.na(language)) %>%
+    dplyr::filter(!is.na(language)) |>
     # Add repository count weight (second most important factor)
-    mutate(
+    dplyr::mutate(
       repo_count_score = log1p(repo_count) * 3.5,
       # Final score = base score (with time weight) + repo count bonus
       final_score = total_score + repo_count_score
-    ) %>%
-    arrange(desc(final_score))
+    ) |>
+    dplyr::arrange(dplyr::desc(final_score))
 
   # Print statistical results
   message("Language statistics:")
@@ -84,8 +68,8 @@ plot_repos <- function(repos_data) {
   cat("\n")
 
   # Calculate percentages and create labels
-  repos_df <- repos_df %>%
-    mutate(
+  repos_df <- repos_df |>
+    dplyr::mutate(
       percentage = round(final_score / sum(final_score) * 100, 1),
       label = paste0(language, " (", percentage, "%)")
     )
diff --git a/R/treemap.R b/R/treemap.R
index 5152c8a..ff25204 100644
--- a/R/treemap.R
+++ b/R/treemap.R
@@ -12,27 +12,6 @@
 # Last Modified: 2025-10-26 11:18:57
 ##
 
-# Load required packages
-if (!require("ggplot2", quietly = TRUE)) {
-  install.packages("ggplot2")
-  library(ggplot2)
-}
-
-if (!require("treemapify", quietly = TRUE)) {
-  install.packages("treemapify")
-  library(treemapify)
-}
-
-if (!require("showtext", quietly = TRUE)) {
-  install.packages("showtext")
-  library(showtext)
-}
-
-if (!require("svglite", quietly = TRUE)) {
-  install.packages("svglite")
-  library(svglite)
-}
-
 #' Generate Treemap Visualization
 #'
 #' Creates a treemap visualization of repository language distribution
@@ -44,16 +23,7 @@ if (!require("svglite", quietly = TRUE)) {
 #' @param json_data Raw repository data
 #' @export
 generate_treemap <- function(username, repos, dir_name, json_data) {
-  # Setup font for visualization
-  tryCatch(
-    font_add("FiraCode", regular = "./assets/FiraCode.ttf"),
-    error = function(e) {
-      warning("Failed to load custom font, using default: ", e$message)
-    }
-  )
-
-  # Enable showtext
-  showtext_auto()
+  font_family <- setup_visualization_font()
 
   # Prepare data for treemap
   total_repos <- nrow(json_data)
@@ -71,34 +41,34 @@ generate_treemap <- function(username, repos, dir_name, json_data) {
   message("Displaying ", nrow(repos_filtered), " languages in treemap (filtered >= 1%)")
 
   # Create the treemap plot
-  p <- ggplot(repos_filtered, aes(
+  p <- ggplot2::ggplot(repos_filtered, ggplot2::aes(
     area = final_score,
     fill = language,
     # label = paste0(language, "\n", percentage, "%\n(", repo_count, " repos)")
     label = paste0(language, "\n", percentage, "%")
   )) +
-    geom_treemap(colour = "white", size = 2) +
-    geom_treemap_text(
-      aes(size = percentage),
+    treemapify::geom_treemap(colour = "white", size = 2) +
+    treemapify::geom_treemap_text(
+      ggplot2::aes(size = percentage),
       place = "centre",
-      family = "FiraCode",
+      family = font_family,
       min.size = 12
     ) +
-    scale_size_continuous(range = c(12, 36)) +
-    labs(
+    ggplot2::scale_size_continuous(range = c(12, 36)) +
+    ggplot2::labs(
       title = "Repository Distribution by Language",
       subtitle = sprintf(
         "GitHub User: %s | Total Repositories: %d | Updated on %s",
         username, total_repos, date_str
       )
     ) +
-    theme_minimal(base_family = "FiraCode") +
-    theme(
-      plot.title = element_text(
+    ggplot2::theme_minimal(base_family = font_family) +
+    ggplot2::theme(
+      plot.title = ggplot2::element_text(
         hjust = 0.5,
         size = 24
       ),
-      plot.subtitle = element_text(
+      plot.subtitle = ggplot2::element_text(
         hjust = 0.5,
         size = 10,
         colour = "#777777"
@@ -120,7 +90,7 @@ generate_treemap <- function(username, repos, dir_name, json_data) {
 
   # Save SVG versions
   for (svg_file in treemap_files$svg) {
-    ggsave(
+    ggplot2::ggsave(
       svg_file,
       plot = p,
       width = 10,
@@ -132,7 +102,7 @@ generate_treemap <- function(username, repos, dir_name, json_data) {
 
   # Save PNG versions
   for (png_file in treemap_files$png) {
-    ggsave(
+    ggplot2::ggsave(
       png_file,
       plot = p,
       width = 10,
diff --git a/README.md b/README.md
index 7fe4e9c..8c5ecaf 100644
--- a/README.md
+++ b/README.md
@@ -38,6 +38,12 @@ export GITHUB_USERNAME="your_username"
 Rscript R/main.R
 ```
 
+### Tests
+
+```bash
+Rscript tests/run_tests.R
+```
+
 ### GitHub Actions
 
 The workflow runs automatically on the 1st of each month, or can be triggered manually via workflow dispatch. Results are committed to the `data` branch with the following structure:
@@ -61,7 +67,11 @@ You can link to these images in your GitHub README or personal website to showca
 ## Requirements
 
 - R >= 4 (tested on 4.5.1)
-- R packages: `httr`, `jsonlite`, `dplyr`, `magrittr`, `showtext`, `ggplot2`, `treemapify`, `svglite`
+- R packages: `httr`, `jsonlite`, `dplyr`, `showtext`, `sysfonts`, `ggplot2`, `treemapify`, `svglite`
+
+```r
+install.packages(c("httr", "jsonlite", "dplyr", "showtext", "sysfonts", "ggplot2", "treemapify", "svglite"))
+```
 
 ## Configuration
 
diff --git a/tests/run_tests.R b/tests/run_tests.R
new file mode 100644
index 0000000..6dfd31b
--- /dev/null
+++ b/tests/run_tests.R
@@ -0,0 +1,80 @@
+#!/usr/bin/env Rscript
+
+source("R/helpers.R")
+source("R/plot.R")
+source("R/pie.R")
+source("R/treemap.R")
+
+run_test <- function(name, code) {
+  tryCatch(
+    {
+      force(code)
+      message("PASS: ", name)
+    },
+    error = function(error) {
+      stop("FAIL: ", name, "\n", error$message, call. = FALSE)
+    }
+  )
+}
+
+run_test("all R files parse", {
+  files <- list.files("R", pattern = "[.]R$", full.names = TRUE)
+  invisible(lapply(files, parse))
+})
+
+fixture <- data.frame(
+  language = c("R", "R", "Go", NA),
+  updated_at = as.character(Sys.Date() - c(0, 365, 0, 0)),
+  stargazers_count = c(10, 0, 1, 100),
+  forks_count = c(2, 0, 0, 100)
+)
+
+run_test("repository statistics are aggregated and ranked", {
+  invisible(capture.output(repos <- suppressMessages(plot_repos(fixture))))
+
+  stopifnot(
+    identical(as.character(repos$language), c("R", "Go")),
+    identical(repos$repo_count, c(2L, 1L)),
+    identical(repos$total_stars, c(10, 1)),
+    identical(repos$total_forks, c(2, 0)),
+    isTRUE(all.equal(sum(repos$percentage), 100)),
+    identical(repos$label, paste0(repos$language, " (", repos$percentage, "%)"))
+  )
+})
+
+run_test("bundled custom font is loaded", {
+  font_family <- suppressWarnings(setup_visualization_font())
+  stopifnot(identical(font_family, "FiraCode"))
+})
+
+run_test("missing custom font falls back to sans", {
+  font_family <- suppressWarnings(setup_visualization_font(tempfile()))
+  stopifnot(identical(font_family, "sans"))
+})
+
+run_test("pie chart and treemap files are generated", {
+  invisible(capture.output(repos <- suppressMessages(plot_repos(fixture))))
+  output_dir <- tempfile("langstat-test-")
+  dir.create(output_dir)
+  on.exit(unlink(output_dir, recursive = TRUE), add = TRUE)
+
+  suppressMessages(generate_pie_chart("test-user", repos, output_dir, fixture))
+  suppressMessages(generate_treemap("test-user", repos, output_dir, fixture))
+
+  today <- format(Sys.Date(), "%Y%m%d")
+  expected_files <- c(
+    paste0("piechart_", today, ".svg"),
+    paste0("piechart_", today, ".png"),
+    "latest.svg",
+    "latest.png",
+    paste0("treemap_", today, ".svg"),
+    paste0("treemap_", today, ".png"),
+    "treemap_latest.svg",
+    "treemap_latest.png"
+  )
+  output_files <- file.path(output_dir, expected_files)
+
+  stopifnot(all(file.exists(output_files)), all(file.size(output_files) > 0))
+})
+
+message("All tests passed.")