function callApi(path, perPage, callback) {
    let xhr = new XMLHttpRequest()
    let apiUrl = `${apiEndpoint}${path}`
    if (perPage >= 1) {
        // add pagination parameter
        apiUrl += `${path.includes("?") ? "&" : "?"}per_page=${perPage}`
    }
    xhr.open("GET", apiUrl)
    xhr.setRequestHeader("Authorization", authHeader)
    xhr.onload = () => {
        if (xhr.status == 200) {
            try {
                let json = JSON.parse(xhr.responseText)
                if (callback) { callback(json) }
            } catch (e) {
                if (e instanceof SyntaxError) {
                    console.error(
                        `Cannot parse response for ${path} as JSON:\n` +
                        xhr.responseText
                    )
                } else { throw e }
            }
        } else {
            console.error(
                `XHR failed when retrieving ${path} (status ${xhr.status}):\n` +
                xhr.responseText
            )
        }
    }
    xhr.send()
}

function syncCanvas() {
    const courses = plasmoid.configuration.courses.split("\n").map(
        // each line in the "courses" config consists of
        // a numeric course id, a space, and a course code
        line => {
            const spaceIndex = line.indexOf(" ")
            return {id: line.slice(0, spaceIndex), code: line.slice(spaceIndex + 1)}
        }
    )
    let courseIndices = {} // reverse look-up table to sort by courses
    for (let i = 0; i < courses.length; i++) {
        courseIndices[courses[i].id] = i
    }

    const showSubmittedAssignments = plasmoid.configuration.showSubmittedAssignments

    // we need user id to check submission status
    callApi("/users/self", 0, user => {
        syncCourses(courses, courseIndices, showSubmittedAssignments, user.id)
    })
}

// fetch asynchronously, but display in this order:
// important -> normal -> finished
// when an activity is both important and finished, important takes priority
function syncCourses(courses, courseIndices, showSubmittedAssignments, userId) {
    announcementsModel.clear()
    assignmentsModel.clear()

    let announcementIndices = {
        important: 0, // actually constant, kept for symmetry
        normal: 0,
        finished: 0,
    }
    let assignmentIndices = {
        important: 0,
        normal: 0,
        finished: 0,
    }

    for (let course of courses) {
        const courseIdx = courseIndices[course.id]
        callApi(`/announcements?context_codes[]=course_${course.id}`, 50, announcements => {
            announcements.forEach(announcement => {
                const info = {
                    type: "announcement",
                    activityId: announcement.id,
                    courseId: course.id,
                    course: course.code,
                    title: announcement.title,
                    url: announcement.html_url,
                    important: plasmoid.configuration.importantAnnouncements
                               .includes(announcement.id.toString()),
                    finished: plasmoid.configuration.finishedAnnouncements
                              .includes(announcement.id.toString()),
                }

                // figure out where we insert it into list
                let idx = 0
                let endIdx = 0 // actually past the end
                if (info.important) {
                    idx = announcementIndices.important
                    endIdx = announcementIndices.normal
                    announcementIndices.normal++
                    announcementIndices.finished++
                } else if (!info.finished) {
                    idx = announcementIndices.normal
                    endIdx = announcementIndices.finished
                    announcementIndices.finished++
                } else {
                    idx = announcementIndices.finished
                    endIdx = announcementsModel.count
                }

                for (; idx < endIdx; idx++) {
                    const annc = announcementsModel.get(idx)
                    if (courseIndices[course.id] < courseIndices[annc.courseId]) {
                        // we are just past the end of this course
                        // insert this announcement here
                        break
                    }
                }
                announcementsModel.insert(idx, info)
            })
        })

        callApi(`/courses/${course.id}/assignments`, 50, assignments => {
            assignments.forEach(assignment => {
                callApi(`/courses/${course.id}/assignments/${assignment.id}` +
                    `/submissions/${userId}`, 0, submission => {
                    const submitted = submission.workflow_state == "submitted" ||
                                      submission.workflow_state == "graded"
                    if (submitted && !showSubmittedAssignments) return // discard this

                    const info = {
                        type: "assignment",
                        activityId: assignment.id,
                        courseId: course.id,
                        course: course.code,
                        title: assignment.name,
                        // if null, use empty string to suppress errors
                        dueAt: assignment.due_at || "",
                        submitted: submitted,
                        url: assignment.html_url,
                        important: plasmoid.configuration.importantAssignments
                                   .includes(assignment.id.toString()),
                        finished: plasmoid.configuration.finishedAssignments
                                  .includes(assignment.id.toString()),
                    }

                    let idx = 0
                    let endIdx = 0
                    if (info.important) {
                        idx = assignmentIndices.important
                        endIdx = assignmentIndices.normal
                        assignmentIndices.normal++
                        assignmentIndices.finished++
                    } else if (!info.finished) {
                        idx = assignmentIndices.normal
                        endIdx = assignmentIndices.finished
                        assignmentIndices.finished++
                    } else {
                        idx = assignmentIndices.finished
                        endIdx = assignmentsModel.count
                    }

                    for (; idx < endIdx; idx++) {
                        const annc = assignmentsModel.get(idx)
                        if (courseIndices[course.id] < courseIndices[annc.courseId]) {
                            break
                        }
                    }
                    assignmentsModel.insert(idx, info)
                })
            })
        })
    }
}