backend/API/Controllers/CostAnalysisController.cs

@@ -0,0 +1,108 @@
+using Extrudex.API.DTOs.PrintJobs;
+using Extrudex.Domain.Interfaces;
+using Microsoft.AspNetCore.Mvc;
+
+namespace Extrudex.API.Controllers;
+
+/// <summary>
+/// Controller for cost analysis endpoints. Provides spool-level
+/// cost breakdowns and aggregated COGS reporting.
+/// </summary>
+[ApiController]
+[Route("api/cost-analysis")]
+public class CostAnalysisController : ControllerBase
+{
+    private readonly ICostPerPrintService _costService;
+    private readonly ILogger<CostAnalysisController> _logger;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="CostAnalysisController"/> class.
+    /// </summary>
+    /// <param name="costService">The cost-per-print calculation service.</param>
+    /// <param name="logger">The logger for diagnostic output.</param>
+    public CostAnalysisController(
+        ICostPerPrintService costService,
+        ILogger<CostAnalysisController> logger)
+    {
+        _costService = costService;
+        _logger = logger;
+    }
+
+    // ── POST /api/cost-analysis/spool ────────────────────────────
+
+    /// <summary>
+    /// Calculates cost breakdowns for all print jobs associated with a specific spool.
+    /// Returns per-job costs plus an aggregated total. Jobs with missing cost data
+    /// include warnings and null cost fields — the endpoint never throws for missing data.
+    /// </summary>
+    /// <param name="request">The request containing the spool identifier.</param>
+    /// <returns>A spool-level cost summary with per-job breakdowns.</returns>
+    /// <response code="200">Returns the spool cost breakdown with per-job details.</response>
+    /// <response code="404">If the spool has no print jobs.</response>
+    [HttpPost("spool")]
+    [ProducesResponseType(typeof(SpoolCostResponse), StatusCodes.Status200OK)]
+    [ProducesResponseType(StatusCodes.Status404NotFound)]
+    public async Task<ActionResult<SpoolCostResponse>> CalculateSpoolCost([FromBody] SpoolCostRequest request)
+    {
+        _logger.LogDebug("Calculating cost breakdown for spool {SpoolId}", request.SpoolId);
+
+        var results = await _costService.CalculateBySpoolAsync(request.SpoolId);
+
+        if (results.Count == 0)
+        {
+            return NotFound(new { error = $"No print jobs found for spool with ID '{request.SpoolId}'." });
+        }
+
+        // Build the spool-level summary
+        var firstResult = results[0];
+        var jobResponses = results.Select(MapCostToResponse).ToList();
+
+        // Aggregate total cost and grams — only include jobs that have a valid cost
+        var calculableJobs = results.Where(r => r.CostPerPrint.HasValue).ToList();
+        var totalCost = calculableJobs.Count == results.Count
+            ? Math.Round(calculableJobs.Sum(r => r.CostPerPrint!.Value), 4)
+            : (decimal?)null;
+
+        var aggregateWarnings = new List<string>();
+        if (calculableJobs.Count < results.Count)
+        {
+            aggregateWarnings.Add(
+                $"{results.Count - calculableJobs.Count} of {results.Count} print jobs have missing cost data. " +
+                "Total cost reflects only jobs with complete data.");
+        }
+
+        var response = new SpoolCostResponse
+        {
+            SpoolId = request.SpoolId,
+            SpoolSerial = firstResult.SpoolSerial,
+            PurchasePrice = firstResult.PurchasePrice,
+            WeightTotalGrams = firstResult.WeightTotalGrams,
+            CostPerGram = firstResult.CostPerGram,
+            TotalGramsConsumed = results.Sum(r => r.GramsDerived),
+            TotalCost = totalCost,
+            JobCount = results.Count,
+            Jobs = jobResponses,
+            Warnings = aggregateWarnings
+        };
+
+        return Ok(response);
+    }
+
+    /// <summary>
+    /// Maps a domain CostPerPrintResult to an API CostPerPrintResponse DTO.
+    /// </summary>
+    private static CostPerPrintResponse MapCostToResponse(CostPerPrintResult r) => new()
+    {
+        PrintJobId = r.PrintJobId,
+        PrintName = r.PrintName,
+        SpoolId = r.SpoolId,
+        SpoolSerial = r.SpoolSerial,
+        MmExtruded = r.MmExtruded,
+        GramsDerived = r.GramsDerived,
+        PurchasePrice = r.PurchasePrice,
+        WeightTotalGrams = r.WeightTotalGrams,
+        CostPerGram = r.CostPerGram,
+        CostPerPrint = r.CostPerPrint,
+        Warnings = r.Warnings
+    };
+}

backend/API/DTOs/PrintJobs/CostPerPrintDtos.cs

@@ -0,0 +1,99 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Extrudex.API.DTOs.PrintJobs;
+
+/// <summary>
+/// Response DTO for cost-per-print calculation. Contains the full cost
+/// breakdown and any warnings about missing or incomplete data.
+/// </summary>
+public class CostPerPrintResponse
+{
+    /// <summary>The print job identifier this result belongs to.</summary>
+    public Guid PrintJobId { get; set; }
+
+    /// <summary>Human-readable name of the print job.</summary>
+    public string PrintName { get; set; } = string.Empty;
+
+    /// <summary>The spool identifier that provided filament.</summary>
+    public Guid SpoolId { get; set; }
+
+    /// <summary>Serial number of the spool.</summary>
+    public string SpoolSerial { get; set; } = string.Empty;
+
+    /// <summary>Total millimeters of filament extruded.</summary>
+    public decimal MmExtruded { get; set; }
+
+    /// <summary>Derived grams consumed for this print.</summary>
+    public decimal GramsDerived { get; set; }
+
+    /// <summary>The spool's purchase price. Null if not recorded.</summary>
+    public decimal? PurchasePrice { get; set; }
+
+    /// <summary>The spool's total weight in grams when full.</summary>
+    public decimal? WeightTotalGrams { get; set; }
+
+    /// <summary>Cost per gram of filament. Null if purchase price or total weight is missing.</summary>
+    public decimal? CostPerGram { get; set; }
+
+    /// <summary>Calculated cost of this print job. Null if cost data is incomplete.</summary>
+    public decimal? CostPerPrint { get; set; }
+
+    /// <summary>
+    /// Warnings about missing or incomplete data. Empty when all data is available
+    /// and the calculation succeeded.
+    /// </summary>
+    public List<string> Warnings { get; set; } = new();
+}
+
+/// <summary>
+/// Request DTO for batch cost calculation by spool. Returns cost breakdowns
+/// for all print jobs associated with the specified spool.
+/// </summary>
+public class SpoolCostRequest
+{
+    /// <summary>The unique identifier of the spool to calculate costs for.</summary>
+    [Required(ErrorMessage = "SpoolId is required.")]
+    public Guid SpoolId { get; set; }
+}
+
+/// <summary>
+/// Response DTO for spool-level cost calculation. Contains cost breakdowns
+/// for all print jobs on the spool, plus a total cost summary.
+/// </summary>
+public class SpoolCostResponse
+{
+    /// <summary>The spool identifier.</summary>
+    public Guid SpoolId { get; set; }
+
+    /// <summary>Serial number of the spool.</summary>
+    public string SpoolSerial { get; set; } = string.Empty;
+
+    /// <summary>The spool's purchase price. Null if not recorded.</summary>
+    public decimal? PurchasePrice { get; set; }
+
+    /// <summary>The spool's total weight in grams when full.</summary>
+    public decimal? WeightTotalGrams { get; set; }
+
+    /// <summary>Cost per gram of filament. Null if cost data is incomplete.</summary>
+    public decimal? CostPerGram { get; set; }
+
+    /// <summary>Total grams consumed across all print jobs on this spool.</summary>
+    public decimal TotalGramsConsumed { get; set; }
+
+    /// <summary>Total calculated cost across all print jobs. Null if any job has missing data.</summary>
+    public decimal? TotalCost { get; set; }
+
+    /// <summary>Number of print jobs included in this calculation.</summary>
+    public int JobCount { get; set; }
+
+    /// <summary>
+    /// Individual cost breakdowns per print job. Jobs with missing data
+    /// will have null cost fields and populated warnings.
+    /// </summary>
+    public List<CostPerPrintResponse> Jobs { get; set; } = new();
+
+    /// <summary>
+    /// Aggregate warnings about missing data across all jobs.
+    /// </summary>
+    public List<string> Warnings { get; set; } = new();
+}

backend/Domain/Interfaces/ICostPerPrintService.cs

@@ -0,0 +1,76 @@
+namespace Extrudex.Domain.Interfaces;
+
+/// <summary>
+/// Service interface for calculating the cost of goods sold (COGS) per print job.
+/// Uses the spool's purchase price and the print job's derived grams consumed
+/// to produce a cost breakdown. Handles missing cost data gracefully by returning
+/// warnings rather than throwing exceptions.
+/// </summary>
+public interface ICostPerPrintService
+{
+    /// <summary>
+    /// Calculates the cost per print for a specific print job.
+    /// </summary>
+    /// <param name="printJobId">The unique identifier of the print job.</param>
+    /// <param name="cancellationToken">Optional cancellation token.</param>
+    /// <returns>
+    /// A <see cref="CostPerPrintResult"/> containing the cost breakdown,
+    /// or warnings if cost data is missing or incomplete.
+    /// </returns>
+    Task<CostPerPrintResult> CalculateAsync(Guid printJobId, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Calculates cost breakdowns for all print jobs associated with a specific spool.
+    /// Useful for spool-level COGS reporting.
+    /// </summary>
+    /// <param name="spoolId">The unique identifier of the spool.</param>
+    /// <param name="cancellationToken">Optional cancellation token.</param>
+    /// <returns>
+    /// A list of <see cref="CostPerPrintResult"/> for each print job on the spool.
+    /// Jobs with missing cost data will include warnings.
+    /// </returns>
+    Task<IReadOnlyList<CostPerPrintResult>> CalculateBySpoolAsync(Guid spoolId, CancellationToken cancellationToken = default);
+}
+
+/// <summary>
+/// Result of a cost-per-print calculation. Contains the cost breakdown
+/// and any warnings about missing or incomplete cost data.
+/// </summary>
+public class CostPerPrintResult
+{
+    /// <summary>The print job identifier this result belongs to.</summary>
+    public Guid PrintJobId { get; set; }
+
+    /// <summary>Human-readable name of the print job.</summary>
+    public string PrintName { get; set; } = string.Empty;
+
+    /// <summary>The spool identifier that provided filament.</summary>
+    public Guid SpoolId { get; set; }
+
+    /// <summary>Serial number of the spool.</summary>
+    public string SpoolSerial { get; set; } = string.Empty;
+
+    /// <summary>Total millimeters of filament extruded.</summary>
+    public decimal MmExtruded { get; set; }
+
+    /// <summary>Derived grams consumed for this print.</summary>
+    public decimal GramsDerived { get; set; }
+
+    /// <summary>The spool's purchase price. Null if not recorded.</summary>
+    public decimal? PurchasePrice { get; set; }
+
+    /// <summary>The spool's total weight in grams when full.</summary>
+    public decimal? WeightTotalGrams { get; set; }
+
+    /// <summary>Cost per gram of filament. Null if purchase price or total weight is missing.</summary>
+    public decimal? CostPerGram { get; set; }
+
+    /// <summary>Calculated cost of this print job. Null if cost data is incomplete.</summary>
+    public decimal? CostPerPrint { get; set; }
+
+    /// <summary>
+    /// Warnings about missing or incomplete data that prevented a full calculation.
+    /// Empty when all data is available and the calculation succeeded.
+    /// </summary>
+    public List<string> Warnings { get; set; } = new();
+}

backend/Infrastructure/Services/CostPerPrintService.cs

@@ -0,0 +1,158 @@
+using Extrudex.Domain.Interfaces;
+using Extrudex.Infrastructure.Data;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+
+namespace Extrudex.Infrastructure.Services;
+
+/// <summary>
+/// Calculates the cost of goods sold (COGS) per print job using the spool's
+/// purchase price and the print job's derived grams consumed.
+///
+/// Formula:
+///   cost_per_gram = purchase_price / weight_total_grams
+///   cost_per_print = grams_derived × cost_per_gram
+///
+/// Handles missing data gracefully — if the spool has no purchase price or
+/// weight recorded, the result includes warnings and null cost fields
+/// instead of throwing exceptions.
+/// </summary>
+public class CostPerPrintService : ICostPerPrintService
+{
+    private readonly ExtrudexDbContext _dbContext;
+    private readonly ILogger<CostPerPrintService> _logger;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="CostPerPrintService"/> class.
+    /// </summary>
+    /// <param name="dbContext">The database context for data access.</param>
+    /// <param name="logger">The logger for diagnostic output.</param>
+    public CostPerPrintService(ExtrudexDbContext dbContext, ILogger<CostPerPrintService> logger)
+    {
+        _dbContext = dbContext;
+        _logger = logger;
+    }
+
+    /// <inheritdoc />
+    public async Task<CostPerPrintResult> CalculateAsync(Guid printJobId, CancellationToken cancellationToken = default)
+    {
+        _logger.LogDebug("Calculating cost per print for job {PrintJobId}", printJobId);
+
+        var job = await _dbContext.PrintJobs
+            .Include(j => j.Spool)
+            .ThenInclude(s => s!.MaterialBase)
+            .FirstOrDefaultAsync(j => j.Id == printJobId, cancellationToken);
+
+        if (job is null)
+        {
+            _logger.LogWarning("Print job {PrintJobId} not found for cost calculation", printJobId);
+            return new CostPerPrintResult
+            {
+                PrintJobId = printJobId,
+                Warnings = new List<string> { $"Print job with ID '{printJobId}' not found." }
+            };
+        }
+
+        return BuildResult(job);
+    }
+
+    /// <inheritdoc />
+    public async Task<IReadOnlyList<CostPerPrintResult>> CalculateBySpoolAsync(
+        Guid spoolId, CancellationToken cancellationToken = default)
+    {
+        _logger.LogDebug("Calculating cost per print for all jobs on spool {SpoolId}", spoolId);
+
+        var jobs = await _dbContext.PrintJobs
+            .Include(j => j.Spool)
+            .ThenInclude(s => s!.MaterialBase)
+            .Where(j => j.SpoolId == spoolId)
+            .OrderByDescending(j => j.CreatedAt)
+            .ToListAsync(cancellationToken);
+
+        if (jobs.Count == 0)
+        {
+            _logger.LogDebug("No print jobs found for spool {SpoolId}", spoolId);
+            return Array.Empty<CostPerPrintResult>();
+        }
+
+        return jobs.Select(BuildResult).ToList();
+    }
+
+    /// <summary>
+    /// Builds a <see cref="CostPerPrintResult"/> from a print job entity.
+    /// Computes cost_per_gram and cost_per_print when all required data is available.
+    /// Populates warnings when data is missing or incomplete.
+    /// </summary>
+    /// <param name="job">The print job entity with Spool navigation loaded.</param>
+    /// <returns>A cost calculation result with breakdown and any warnings.</returns>
+    private CostPerPrintResult BuildResult(Domain.Entities.PrintJob job)
+    {
+        var warnings = new List<string>();
+        var spool = job.Spool;
+
+        // Map what we always have
+        var result = new CostPerPrintResult
+        {
+            PrintJobId = job.Id,
+            PrintName = job.PrintName,
+            SpoolId = job.SpoolId,
+            SpoolSerial = spool?.SpoolSerial ?? string.Empty,
+            MmExtruded = job.MmExtruded,
+            GramsDerived = job.GramsDerived,
+        };
+
+        // Guard: spool must be loaded
+        if (spool is null)
+        {
+            warnings.Add("Spool data is not available for this print job.");
+            result.Warnings = warnings;
+            return result;
+        }
+
+        // Capture purchase price
+        result.PurchasePrice = spool.PurchasePrice;
+        result.WeightTotalGrams = spool.WeightTotalGrams;
+
+        // Check for missing purchase price
+        if (!spool.PurchasePrice.HasValue)
+        {
+            warnings.Add(
+                "Spool purchase price is not recorded. Cost calculation requires a purchase price on the spool.");
+        }
+
+        // Check for zero or negative weight — prevents division by zero
+        if (spool.WeightTotalGrams <= 0)
+        {
+            warnings.Add(
+                "Spool total weight is zero or not recorded. Cost calculation requires a positive weight_total_grams on the spool.");
+        }
+
+        // Check for zero grams derived
+        if (job.GramsDerived <= 0)
+        {
+            warnings.Add(
+                "Derived grams consumed is zero. Ensure mm_extruded, filament diameter, and material density are recorded for this print job.");
+        }
+
+        // If all data is present and valid, compute the cost
+        if (spool.PurchasePrice.HasValue && spool.WeightTotalGrams > 0 && job.GramsDerived > 0)
+        {
+            var costPerGram = spool.PurchasePrice.Value / spool.WeightTotalGrams;
+            result.CostPerGram = Math.Round(costPerGram, 6);
+            result.CostPerPrint = Math.Round(job.GramsDerived * costPerGram, 4);
+
+            _logger.LogDebug(
+                "Cost calculated for job {PrintJobId}: {GramsDerived}g × {CostPerGram:C}/g = {CostPerPrint:C}",
+                job.Id, job.GramsDerived, result.CostPerGram, result.CostPerPrint);
+        }
+        else
+        {
+            _logger.LogDebug(
+                "Cost calculation incomplete for job {PrintJobId}: missing data (warnings: {WarningCount})",
+                job.Id, warnings.Count);
+        }
+
+        result.Warnings = warnings;
+        return result;
+    }
+}

backend/Program.cs

@@ -50,6 +50,9 @@ builder.Services.AddSwaggerGen(c =>
 // ── QR Code Generation ──────────────────────────────────────
 builder.Services.AddSingleton<IQrCodeService, QrCodeService>();
 
+// ── Cost Per Print Calculation ─────────────────────────────
+builder.Services.AddScoped<ICostPerPrintService, CostPerPrintService>();
+
 // ── FluentValidation ──────────────────────────────────────
 // Registers all validators from the API assembly into DI.
 builder.Services.AddValidatorsFromAssembly(Assembly.GetExecutingAssembly());